m 


SMI  I      i$A 


AN  INTRODUCTION  TO  THE  PHYSICS 
AND  CHEMISTRY  OF  COLLOIDS 


of  Chemical  Research  and  Engineering. 


Edited  by  W.  P.  DREAPER,  F.I.C. 


SURFACE  TENSION  AND  SURFACE  ENERGY,  AND 
THEIR  INFUENCE  ON  CHEMICAL  PHENOMENA. 

I'.y   Rv  S.  WILLOWS.    M.A.,    D.Sc..   and    E.    HATSCIIKK.    With    17 
Illustrations.        2s.  6d.  net. 

NOTES  ON  CHEMICAL  RESEARCH. 

Hy  W.  I».  DREAl'ER.  F.I.C.        2s.  6d.  net. 

MOLECULAR   PHYSICS. 

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CHEMICAL  ENGINEERING.  NOTES  ON  GRINDING, 
SIFTING,  SEPARATING  AND  TRANSPORTING 
SOLIDS. 

By  J.   W.    HINCHLEY.   A.RS.M.,   WH.Sc.      With    70    Illustrations. 
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('F    CHEMICAL    RESEARCH   .I.V/)    EXGIXEEKIXG 


AN     INTRODUCTION 


To    THK 


PHYSICS  AND  CHEMISTRY 
OF    COLLOIDS 


EMIL    HATSCHEK 


S7<:C(>.\7>    EDITIOX 
With    17    Illustrations 


PHILADELPHIA  : 

P.  BLAKISTON'S  SON 

1012,    WALNUT    STREET 
1916 


Co. 


Printed  in  Grcnl  Britain. 


PREFACE 

TO    SECOND    EDITION, 


In  preparing  the  second  edition  of  this  small 
work  the  author  has  thought  it  advisable  to  preserve 
its  character  as  a  brief  introduction,  which  may  claim 
to  have  established  its  usefulness,  rather  than  to 
expand  it  into  a  text  book.  Apart  from  small 
corrections  the  only  substantial  addition  to  the  text 
of  the  first  edition  consists  therefore  in  an  appendix 
on  experimental  technique.  In  view  of  the  unfamiliar 
nature  of  many  of  the  procedures  and  devices  em- 
ployed in  the  investigation  of  colloids,  and  in  the 
absence  of  any  work  devoted  to  colloidal  laboratory 
practice  even  the  brief  directions  given  may  be  of 
assistance  to  the  student. 

EMIL    HATSCHKK. 

456180 


PREFACE 

TO     FIRST     EDITION 


The  present  work  is  a  slightly  enlarged  reprint  of 
a  series  of  articles  published  in  The  Chemical  \Vorhi, 
which  in  their  turn  were  based  on  a  course  of  ten 
lectures  delivered  at  the  Sir  John  Cass  Technical 
Institute  to  students  of  very  varied  attainments 
and  interested  in  every  branch  of  chemistry  and  of 
chemical  industry.  The  book  accordingly  does  not 
aim  at  a  completeness  precluded  alike  by  its  compass 
and  the  extremely  vigorous  growth  of  the  subject, 
but  is  only  intended  to  introduce  readers  with  a 
reasonable  knowledge  of  physics  and  chemistry  to 
the  fundamental  facts  and  methods  of  a  branch  of 
physical  chemistry,  on  the  importance  of  which  it 
is  hardly  necessary  to  insist. 

Certain  features*  in  the  selection  of  subjects  and 
in  the  order  of  presenting  them,  which  will  be  apparent 
to  readers  familiar  with  the  existing  literature,  are 
due  to  a  vivid  recollection  of  the  difficulties 
experienced  by  the  author  iii  his  first  studies,  and  the 
desire  to  spare  the  student  as  many  of  these  as  appear 
avoidable.  Those  desirous  of  ampler  and  more 
detailed  information  are  referred  to  Wolfgang 
Ostwald's  "  Grundriss  der  Kolloidchemie"  and 
H.  Freundlich's  "  Kapillarchemie"  English  trans- 
lations of  which  are  urgently  required. 

EMIL  HATSCHKK. 

London,  W13. 


CONTENTS. 


PAGE 

PREFACE  TO  SKCOXD  EDITION  IV 

I'RKKACK    TO    FlKST    KlJlTIOX  V 

CHAPTER  I'.  1 

History  of  subject.  The  work  o!"  Thomas  Graham  and 
of  earlier  investigators.  Modern  development.  Generality 
of  the  colloidal  state.  Artificially  prepared  inorganic  and 
natural  organic  colloids.  Diffusion  through  membranes 
and  osmotic  pressure.  Adsorption  by  colloids.  Diagnosis 
of  colloids,  principally  by  dialysis  ;  various  forms  of 
dialysers.  Preparation  of  several  typical  sols.  Character- 
istic behaviour  :  appearance  in  Tyndall  cone  and  effect  of 
electrolytes. 

CHAPTER  II.    -  II 

Methods  of  investigation  as  applied  to  elucidation  of 
peculiarities  of  colloidal  state.  Filtration,  ultra-filtration. 
Si/,es  of  pores  in  ultra-filters.  Tyndall  cone,  size  of 
particles  as  compared  with  wave  length  of  light.  Limits 
of  microsc  pic  visibility.  Slit  ultra-microscope  and 
calculation  of  si/e  of  particles.  I'ltra-condensers.  Sols 
as  systems  of  two  phases.  Two  principle  classes  of 
colloids.  Grounds  of  classification. 

CHAPTER  III.  21 

Viscosity  of  liquids  ;  definition  of  viscosity  coefficient. 
Striking  differences  between  two  types  of  colloids  in 
respect  of  viscosity.  Reasoning  by  analogy  from  coarser 
systems.  Slight  increase  of  viscosity  with  solid  disperse 
phase  ;  large  increase  possible  only  with  liquid  disperse 
phase.  Mathematical  evidence.  Suspensoids  and  emul- 
soids.  Properties  of  particles  in  suspension.  Stokes's 
formula.  Discussion  and  numerical  examples.  Small 
size  alone  insufficient  to  explain  properties  of  sols. 
Brownian  movement.  Mathematical  and  experimental 
investigation.  Electric  charge  and  stability.  Demonstra- 
tion of  charge  by  U-tube  and  by  microscopic  method. 


nil. 

PAGE 

CHAPTER  IV.  -        .>! 

Methods  of  preparing  suspcnsoicl  sols.  Dilution.  Disin- 
tegration methods  .  Bredig's  and  Svedberg's  electrical 
methods.  Ku/.el's  method.  Direct  sol  formation  from 
solid  metals.  Peptisation.  Electrolyte  coagulation. 
Influence  of  valancy.  Adsorption  of  equivalent  amounts. 
Protective  colloids.  Gold  figures. 

CHAPTER  V.  >s 

Emulsions.  Pure  oil-water  emulsions.  Effect  of  increasing 
volume  of  disperse  phase,  and  unlimited  phase  ratio. 
Emulsifying  agents  and  effect  on  interl'acial  tension. 
Donnan's  pipette.  Separation  of  emulsions.  Emulsoids. 
Silicic  acid.  Coagulation  and  gel  formation.  Organic 
einulsoids.  Gelatine  and  agar.  Hysteresis.  Viscosilv 
of  gelatine  sols.  Anomalies  explained  by  presence  <>l 
t\vo  liquid  phases. 

CHAPTER  VI.  47 

Emulsoids  Continued).  Albumin.  lle.it  coagulation. 
Electrolyte  coagulation.  Hofmeister  series.  General 
significance  of  series.  Effect  on  compressibility.  Other 
einulsoids:  casein,  gum  arabic.  cellulose  and  nitrocellulose 
sols.  Electrical  properties  and  ultra-microscopic  appeal - 
ance.  Transition  to  true  solutions.  Soaps.  Tannin. 
Peptones.  Dye  stuffs. 

CHAPTER  VII.  54 

Gels.  Xo  loss  of  water.  Probability  of  structure.  Elastic 
and  rigid  gels.  Preparation  and  dehydration  of  silicic- 
acid  gel.  Van  Hemmelen's  curve.  Continuity  of  process. 
Adsorption  compounds.  Optical  changes  in  silicic  acid 
gel.  Elastic  gels  :  swelling.  Total  compression  :  methods 
of  demonstration.  Liberation  of  heat.  Swelling  of  gel 
under  pressure.  Physical  constants  of  elastic  gels. 
Structure  and  nature  of  phases.  Hiitschli's  microscopic 
investigations.  Objections.  Diffusion  as  evidence  oi 
structure.  Bechhold  and  Ziegler's  experiments.  Reactions 
in  gels.  Liesegang  rings.  Si/e  and  shape  of  reaction 
products.  Gels  from  markedly  crystalline  substances. 

CHAPTER  VI II.  <>8 

Changes  of  concentration  at  boundary  surfaces.  Familiar 
examples.  Surface  tension  and  energy.  Surface  tensions 
on  solid  surfaces.  Indirect  methods  of  investigation. 
Changes  in  concentration  accompanied  by  decrease  of 
surface  energy.  Condensation  of  gases  on  mercurv. 
Surface  concentration  of  amyl  of  alcohol  solutions. 
\Villard  Gibb's  theorem.  Discussion. 


PAGE 

CHAPTER  IX.  74 

Discussion  (continued}.  Adsorption  from  mixtures. 
Effect  of  surface.  Equilibrium.  Wilhelm  Ostwald's 
and  Freundlich's  crucial  experiments.  Equation  of 
adsorption  isotherm.  Discussion  of  equation  and  num- 
erical example.  Values  of  exponent.  Adsorption  from 
different  solvents.  Freundlich's  theory  of  electrolyte 
precipitation.  Capillary  analysis.  Adsorption  proved  by 
isotherm.  Adsorption  and  chemical  changes.  Ex- 
traction. Extraction  curves  and  construction  of  corres- 
ponding isotherms.  Electrical  adsorption.  Adsorption 
effects  inseparable  from  all  phenomena  in  disperse 
systems. 

CHAPTER  X  -        87 

General  conclusions  and  points  of  view  to  be  drawn 
from  preceding  chapters.  Suspension  and  suspensoids. 
Liquid  disperse  phase  introduces  possibility  of  deforma- 
tion and  unlimited  phase  ratio.  Transfer  of  continuous 
into  disperse  phase.  Gels  as  systems  with  solid  contin- 
uous phase.  Adsorption.  Application  of  colloidal 
science.  Theoretical.  Practical  twofold  :  principally 
in  providing  method  of  investigating  processes  used 
and  controlled  empirically.  Conclusion. 

A  PP  EX  I)  IX     -  -  •  -        93 

Experimental  methods  of  examination  and  preparation. 
Tyndall  cone  ;  analyzing  device  for  examining  it.  Ultra- 
condenser.  Dialysis  ;  collodion  thimbles.  Ultra-filter 
and  collodion  membranes  used  with  it.  Electrolyte 
coagulation  and  cataphoresis  tests.  Preparation  of 
gelatine,  agar  and  silicic  acid  sols.  Adsorption  experi- 
ments. 


SUBJECT-MATTER  INDEX  103 

NAME  IXDKX-  -  -  -        107 


AN  INTRODUCTION  TO  THE 

PHYSICS    AND    CHEMISTRY 

OF    COLLOIDS. 


CHAPTER  I. 

THE  first  systematic  investigation  of  our  subject,  and 
the  name  "  Colloids,"  are  due  to  Thomas  Graham,  whose, 
results  were  published  in  a  number  of  papers  between! 
1861  and  1864,  some  of  which  have  recently  been  re- i 
published  and  included  by  Wilhelm  Ostwald  in  the  series 
of  Classics  of  the  exact  Sciences."  His  discoveries  may 
be  briefly  summarised  under  two  heads.  In  studying" 
dialysis,  i.e.,  the  diffusion  of  dissolved  substances  through 
organic  membranes  like  parchment  into  the  pure  solvent, 
he  found  that  some  of  them  passed  freely  through  the 
membrane  into  the  surrounding  solvent,  while  a  number 
of  others  failed  to  do  so,  or  diffused  at  an  extremely  slow 
rate.  Generally  speaking,  the  former  wrere  bodies  which 
were  known  to  crystallise,  while  the  latter,  e.g.,  glue, 
gelatine,  gum-arabic,  etc.,  were  known  only  in  the  amor- 
phous condition.  Graham  accordingly  divided  soluble 
bodies  into  two  classes  :  Crystalloids  and  Colloids  (from 
colla,  glue). 

The  second  discovery  made  by  Graham,  in  prosecuting 
these  researches,  was  that  a  number  of  substances  gener- 

1 


OK  THOMAS   GRAHAM. 


ally  considered  as  insoluble  could,  by  appropriate  methods, 
be  obtained  in  what  at  first  sight  appeared  to  be  real 
solutions.  These  methods  will  be  referred  to  later  on,  but 
it  may  be  mentioned  here  that  Graham  obtained,  and  veiy 
carefully  investigated,  such  solutions  of  silicic  acid, 
tungstic  acid,  chromium-aluminium-and  ferric  hydroxide. 
Most  of  these  appeared  to  the  eye  like  true  solutions,  i.e.. 
they  were  perfectly  clear  and  apparently  homogeneous. 
They  did,  however,  not  diffuse  through  parchment,  and  on 
account  of  this  property  they  were  called  by  Graham 
colloidal  solutions,"  or  simply  "  sols,"  which  term  has 
now  become  generally  accepted.  These  sols  showed  a 
further  striking  peculiarity,  which  distinguished  them 
sharply  from  true  solutions,  such  as,  say,  solutions  of 
sodium  chloride  or  copper  sulphate,  inasmuch  as 
extremely  small  additions  of  electrolytes,  and  such  as  did 
not  react  at  all  with  the  dissolved  substances,  caused 
radical  alterations  in  the  condition  of  the  solutions.  A 
trace  of  carbon  dioxide,  e.g..,  caused  the  silicic  acid  sol  to 
set  to  a  translucent  jelly,  while  a  small  addition  of 
sodium  sulphate  would  precipitate  the  ferric  hydroxide  as 
a  flocculent  mass.  Obviously  these  were  phenomena 
quite  different  from  known  reactions  of  a  purely  chemical 
nature,  and  the  sols  had  to  be  considered  as  systems 
differing  from  ordinary  true  solutions. 

While  Graham's  investigations  were  the  first  ones 
directed  systematically  towards  the  production  of  sols  and 
the  study  of  their  characteristic  properties,  numerous 
earlier  instances  of  sol  formation,  i.e.,  the  production  of 
apparent  solutions  of  substances  known  as  insoluble,  are 
not  wanting.  Wohler  found  in  1839  that  silver  citrate 
heated  in  a  stream  of  hydrogen  left  a  residue  which 
dissolved  in  water  with  a  red  colour ;  this  solution,  which 


EARLIER  OBSERVATIONS  AND  MODERN  DEVELOPMENTS.  3 

lie  took  to  be  a  silver  subcitrate,  has  since  been  proved 
to  be  an  impure  silver  sol.  In  1857  Faraday  obtained  a 
red  liquid  by  reducing  a  very  dilute  solution  of  gold 
chloride  with  a  few  drops  of  ethereal  phosphorus  solution, 
and  expressed  the  opinion  that  the  gold  was  suspended  in 
the  liquid  in  a  state  of  extremely  fine  distribution.  In 
the  fifties  sols  of  silicic  acid,  aluminium  hydroxide  and 
ferric  hydroxide  were  prepared  respectively  by  Kiihn, 
Crum  and  Pean  de  St.  Gilles,  and  some  very  much 
earlier  instances  have  recently  been  brought  to  light  by 
The  Svedberg,  Prof.  Walden,  and  others. 

Since  Graham's  time  the  whole  subject  has  been 
studied  by  an  ever-increasing  number  of  investigators,, 
both  physicists  and  chemists,  with  new  methods  and 
appliances,  the  most  important  of  which  has  been  the 
"Ultra-microscope,"  invented  in  1905  by  R.  Zsigmondy 
and  H.  Siedentopf,  which,  even  in  that  short  time,  has 
enormously  enlarged  our  knowledge  of  colloidal  solutions. 
The  principal  step  forward  in  theory  has  been  the  proof, 
now  quite  conclusive,  that  *'  colloids,"  in  Graham's  sense, 
are  not  a  definite  class  of  substances,  but  that  by 
suitable  methods  a  very  large  number  of  bodies  can  be 
prepared  in  a  colloidal  condition,  which  thus  presents 
itself  as  a  state,  not  as  form  of  matter.  Thus,  e.g.,  sodium 
chloride  is  certainly  a  very  well-defined  crystalline 
substance  ;  yet  a  colloidal  solution  of  sodium  chloride  in 
petroleum  ether  has  been  prepared  by  Paal.  Most  of  the 
metals  have  been  obtained  as  sols,  and  the  silver  and 
mercury  sols  are  made  commercially  and  are  used  in 
medicine.  Even  the  alkali  metals  have  been  obtained  in 
the  colloidal  state  by  the  use  of  organic  solvents,  low 
temperatures  and  experimental  arrangements  of  great 
ingenuity.  Similarly  a  large  number  of  sols  of  oxides, 


4  VARIOUS  TYPES  OF  SOLS    AND 

hydroxides  and  sulphides  are  known,  and  especially  the 
latter  have  been  used  largely  in  several  classical  investiga- 
tions,, to  which  we  shall  have  occasion  to  refer  later  on. 

All  these  sols  are  laboratory  products.,  prepared  by 
certain  well-defined  methods  and  with  numerous 
precautions.  They  are  in  general  very  dilute,  rarely 
containing  more  than  fractions  of  one  per  cent.,  and 
either  quite  clear  or  slightly  opalescent.  Many  of  them,, 
e.g.,  the  gold  sols,  are  beautifully  coloured  and  show 
pseudo-fluorescence,  i.e.,  the  colour  in  transmitted  light  is 
different  from  that  in  reflected  light.  They  all  have  one 
characteristic  in  common :  on  addition  of  varying,  but 
always  small,  quantities  of  an  electrolyte  they  undergo  a 
marked  and  irreversible  change,  the  solid  matter  being 
precipitated  or  the  whole  liquid  setting  to  a  jelly. 

In  striking  contrast  to  these  artificially  prepared 
products  there  exists  a  large  group  of  substances,  which 
can  be  dissolved  at  once  without  special  methods  to  form 
colloidal  solutions,  and  this  comprises  such  important 
organic  bodies  as  gelatine,  albumin,  starch,  agar,  etc. 
These  differ  a  good  deal  from  one  another  in  their 
behaviour.  Some,  like  gelatine,  form  solutions  above  a 
certain  temperature  ;  when  this  falls  below  a  limit 
depending  on  the  concentration,  the  solution  sets  to  a 
jelly,  which  can  be  "  melted  "  again,  and  this  can  be — 
with  certain  precautions — repeated.  On  the  other  hand, 
egg  albumin  is  soluble  at  ordinary  temperatures,  but  on 
heating  coagulates  to  a  mass,  which  is  now  insoluble. 
Gum-arabic,  to  name  a  third  substance  belonging  to  this 
class,  forms  a  viscous  liquid  which  does  not  set  to  jelly  on 
cooling  nor  coagulates  on  heating.  Similarly  varied  is 
the  behaviour  of  these  organic  substances  towards 
electrolytes/but  they  all  share  with  the  inorganic  sols  the 


THEIR  MOST  GENERAL    PROPERTIES.  5 

one  characteristic  property  of  not  diffusing  through  a 
parchment  membrane. 

In  this  class  too,  it  is  not  necessary  that  the  solvent 
should  be  water,  and  a  number  of  solutions  have  lately 
assumed  very  great  industrial  importance  in  which  this  is 
not  the  case.  Thus  it  is  generally  known  that  cellulose 
dissolves  to  a  viscous  solution  in  Schweizer's  reagent — 
copper  oxide-ammonia — and  this  cellulose  sol  is  one  of 
the  materials  used  in  the  production  of  artificial  silk. 
The  solutions  of  nitre-cellulose  in  various  solvents,  such  as 
ether-alcohol,  acetone  and  acetic  acid,  are  equally  well 
known,  and  form  the  starting  material  in  the  production 
of  artificial  silk,  photographic  films,  etc.  Recent 
experiments  with  solutions  ot  nitro-cellulose  in  acetone, 
which  were  dialysed  against  pure  acetone,  have  shown 
that  these  sols  also  share  the  general  characteristics  of 
the  aqueous  sols,  inasmuch  as  they  do  not  diffuse. 

In  close  connection  with  this  property  is  another 
quite  general  feature  of  colloidal  solutions  :  that  they  do 
not  show  any  appreciable  osmotic  pressure,  or,  in  other 
words,  no  raising  of  the  boiling  or  lowering  of  the 
freezing  point.  Whether  this  absence  of  osmotic  pressure 
is  due  to  the  large  size  of  their  molecules  and  high 
molecular  weights,  we  shall  have  to  discuss  later  in 
considering  the  theory  of  colloidal  solutions  more  in 
detail. 

One  other  property  of  colloids,  which  shows  itself 
particularly  strikingly  in  their  '  gels  " — as  Graham  called 
the  products  obtained  by  the  coagulation  of  sols — is  their 
capacity  for  taking  dissolved  substances  out  of  solution 
and  retaining  them  often  with  very  great  tenacity.  This 
phenomenon,  which  is  of  the  greatest  importance  in 
nature  and  in  many  industries,  is  now  generally  called 


6  PREPARATION   AND  IDENTIFICATION  OF  SOLS. 

"  Adsorption,"  and  lias  become  the  subject  of  most 
extensive  and  careful  investigation.  Instances  will  occur 
at  once  to  our  readers.  Specially  striking  is  the  power  of 
"selective  adsorption"  (the  term  explains  itself)  possessed 
by  many  substances  and  organisms.  Thus  we  should 
probably  be  quite  ignorant  of  the  existence  of  one 
important  element,  iodine,,  and  should  certainly  not  be 
able  to  use  it  freely,  if  it  were  not  for  the  selective 
adsorption  exerted  by  a  number  of  seaweeds,  as  iodine 
cannot  be  shown  to  be  present  in  sea-water  by  any  of  the 
usual  tests. 

Before  entering  upon  the  closer  study  of  colloidal 
solutions,  it  may  be  well  to  answer  two  questions,  which 
will  naturally  suggest  themselves  to  the  reader  who  has 
had  no  practical  experience  of  the  subject  :  how  is  it 
possible  to  tell  whether  a  given  solution  contains  colloids  ? 
and  how  can  a  few  typical  inorganic  sols  (the  organic 
ones  are  more  or  less  well  known)  be  convenient!}' 
prepared  ? 

As  regards  the  first  question,  the  most  direct  method 
— apart  from  ultra-microscopic  examination,  which 
requires  expensive  apparatus  and  considerable  expertness, 
and  will  be  dealt  with  later — of  differentiating  between 
true  and  colloidal  solutions  is  still  dialysis.  This  may  be 
carried  out  in  a  number  of  different  appliances.  Graham's 
original  dialyser,  which  is  shown  in  text-books  and  price- 
lists  with  that  persistence  with  which  certain  illustrations 
reappear  long  after  they  have  become  obsolete,  has  the 
drawbacks  of  small  surface  and  of  being  difficult  to  get 
tight.  It  can,  however,  be  easily  improvised,  as  it 
consists  simply  of  a  shallow  open  cylinder,  over  one  end 
of  which  the  parchment  paper  membrane  is  tied.  The 
solution  to  be  examined  is  then  filled  into  this  cylinder, 


DIALYSIS. 


which  is  placed  in  a  larger  vessel.,  filled  to  the  same  level 
with  the  pure  solvent,  generally,  of  course,  distilled  water. 

A  very  large  surface  is  obtained,  and  the  tying  of  the 
membrane  is  avoided  by  the  use 
of  parchment  tube — sometimes 
inelegantly  called  "  sausage  skin  " 
dialysers.  These  can  be  obtained 
in  various  diameters  and  are 
inexpensive,  but  frequently  defec- 
tive. It  is  therefore  advisable, 
before  proceeding  to  actual  work, 
to  select  a  piece  free  from  defects 
by  hanging  up  a  length  in  U-shape 
and  filling  with  distilled  water. 
If  no  leak  shows  itself  the  piece 
may  then  be  emptied,  hung  up 
again  in  the  shape  of  a  U  (Fig.  l) 
in  a  tall  cylinder  and  filled  with  the 
solution  under  examination.  The 
cylinder  is  then  filled  with  the 
solvent,  or,  preferably,  a  constant 
slow  circulation  is  established  by 
running  the  solvent  in  continuously 
and  syphoning  it  off  from  the 
bottom. 

For  examining  small  quantities 
only,  by  far  the  most  convenient 
method  is  the  use  of  the  diffusion  FlG-  i- 

shells  made  by  Schleicher  &  Schiill.  These  are  seamless 
thimbles  of  stout  parchment  paper,  which  can  be  obtained 
in  several  sizes. 

The  simplest  way  to  use  one  is  to  fill   it  with  the 
liquid   to   be   dialysed,  and   then   to   place  it   in  a  small 


n 
1 

^ 

*s  — 

\ 

V 

j 

DIALYSIS  :  GOLD  SOLS. 


Erlenmeyer  flask  filled  with  water,  which  is  changed  as 
occasion  requires.  (Fig.  2.) 

In  all  cases  the  dialysis  may  be  considered  complete 
when  the  outside  water  remains  pure — which  may,  of 
course,  be  tested  by  suitable  reactions  or  conductivity 
measurements.  Whatever  residue  is  then  left  in  the 
dialyser  is  in  colloidal  solution. 

As  regards  the  second  query,  exact  instructions  for 
preparing  a  number  of  sols — all  of  them  typical — are 
given  in  the  following.  To  avoid  failures  it  is  absolutely 

essential  to  observe  scrupu- 
lous cleanliness.  The  water 
should  be  freshly  distilled,  if 
necessary  re-distilled  with 
a  hard  glass  condenser.  All 
vessels  used  should  be  of 
Jena  or  other  resistance, 
glass,  preferably  new,  but  in 
any  event  cleaned  with 
nitric  or  chromic  acid. 

Gold  .vo/,y.  —  Two  cc. 
of  the  commercial  1  per 
cent,  gold  chloride  solution 
is  diluted  with  100  cc.  of 

distilled  water.  0.5  gr.  of  tannin — the  "  tannic  acid, 
pure"  of  commerce — is  dissolved  in  100  cc.  of  water.  If 
both  solutions  are  heated  and  about  one  part  of  the 
tannin  solution  added  to  three  parts  of  the  gold  chloride 
solution,  a  clear  red  gold  sol  results.  If  equal  portions  of 
the  solutions  are  mixed  cold,  the  sol  produced  is  purple  to 
blue.  In  both  cases  the  reaction  takes  about  a  minute  to 
complete  itself.  These  sols  are  very  stable  and  can  be 
kept  for  some  time  :  as  they  contain  tannin  in  excess 


Km.  2. 


VARIOUS  SOLS.  9 

they  are.,  however,  like  pure  tannin  solutions,  liable  to  go 
mouldy. 

A  great  range  of,  much  less  stable,  sols  can  be 
obtained  by  using  instead  of  tannin  one  of  the  photo- 
graphic developing  agents  as  reducer,  e.g.,  hydrokinone, 
or  pyrogallol,  about  one  part  in  500  of  water.  The  latter 
occasionally  gives  a  fine  sol,  blue  in  transmitted  and 
brick-red  in  reflected  light. 

Silver  sols. — These  are  much  less  stable,  but  one 
fairly  satisfactory  in  this  respect  may  be  prepared  in  the 
following  way  : — To  5  cc.  of  1  per  cent,  silver  nitrate 
solution  add,  drop  by  drop,  ammonia,  until  the  precipitate 
just  disappears,  and  then  dilute  with  100  cc.  of  water, 
If  equal  volumes  of  this  solution  and  of  the  tannin 
solution  just  described  are  mixed,  a  silver  sol  results, 
which  is  clear  and  brown  in  transmitted  light,  but  often 
shows  a  greenish  colour  in  reflected  light,  resembling 
certain  petroleums. 

Antimony  bisulphide  sol. — Dissolve  1  gramme  of 
potassium-antimony  tartrate  ("tartar  emetic  ")  in  100  cc., 
of  water.  Make  a  dilute  ammonium  sulphide  solution, 
about  one  part  of  the  strong  "  sulphide  to  50  parts  of 
water,  and  mix  equal  volumes  of  the  two  solutions.  The 
colour  gradually  changes  to  yellow  or  orange.  This  sol, 
like  the  preceding  ones,,  may  then  be  purified  by  dialysis. 

Arsenic  bisulphide  soL — Boil  2  grammes  of  arsenious 
acid  (white  arsenic)  for  a  few  minutes  with  150  cc.  of 
water,  then  filter  and  allow  to  cool.  If  hydrogen  sulphide  is 
passed  through  the  cold  solution,  it  gradually  turns  orange, 
with  a  strong  greenish  surface  colour  in  reflected  light. 

Ferric  hydroxide  sol. — This  sol  is  very  easily  obtained 
in  the  following  manner  :  heat  about  500  cc.  of  water  to 
boiling  in  a  beaker  or  Erlenmeyer  flask.  While  it  is 


10  TYNDALLCONE;  EFFECT  OF  ELECTROLYTES 

boiling  add  about  5  cc.  of  a  33  per  cent,  solution  of  ferric 
chloride  (which  has  been  filtered,  if  necessary).  The 
colour  at  once  changes  to  a  beautiful  brownish-red,  the 
sol  being  perfectly  clear  and  very  stable.  It  can  be  kept 
for  wreeks  without  any  special  precautions,  but  may  be 
dialysed  to  remove  the  free  hydro-chloric  acid  formed  by 
the  dissociation. 

These  sols  have  been  selected  as  being  very  easy  to 
prepare  and  as  showing  all  the  typical  qualities  of 
colloidal  solutions.  Two  of  these  may  very  easilj-  be 
studied :  the  appearance  with  "  Tyndall "  illumination 
and  the  behaviour  towards  'electrolytes.  To  show  the 
former,  the  sol  is  placed  in  a  glass  vessel  with  plane 
parallel  sides,  and  a  strong  beam  of  light — the  light  of  a 
small  arc  or  a  Nernst  lamp  concentrated  by  a  lens  does 
very  well — projected  through  the  liquid.  This,  if  looked 
at  from  the  side  against  a  dark  background,  will  appear 
turbid,  with  a  greenish  sheen  in  the  case  of  the  gold  and 
silver  sol. 

As  regards  the  behaviour  of  the  sols  towards 
electrolytes,  it  is  only  necessary  to  add  a  few  drops  of  any 
salt  solution — preferably  barium  or  calcium  chloride  in 
the  case  of  the  metal  and  sulphide  sols,  and  sodium 
sulphate  for  the  ferric  hydroxide  sol — to  see  a  marked 
change  at  once.  The  solutions  become  turbid,  change 
colour,  and  after  some  time  the  dissolved  substance  will 
be  found  to  have  settled  out.  This  may  take  some  hours 
with  the  gold  and  silver  sols,  but  if  the  sodium  sulphate 
is  added  to  the  ferric  hydroxide  sol  while  the  solution  is 
still  hot,  the  ferric  hydroxide  will  come  down  as  a 
flocculent  mass  in  a  few  minutes. 

Further  directions  for  the  preparation  and  examina- 
tion of  sols  will  be  found  in  the  appendix. 


CHAPTER   II. 

IN  the  preceding  chapter  some  general  charac- 
teristics of  colloidal  solutions,  such  as  their  appearance 
with  suitable  illumination  and  their  behaviour  on  the 
addition  of  electrolytes,  have  been  referred  to,  and 
one  method  of  investigating  them — dialysis — has  been 
dealt  with  in  some  detail.  It  now  remains  to  describe 
the  other  methods  applied  to  the  study  of  colloids  in 
more  recent  times,  and  to  see  what  conclusions  as  to 
the  nature  of  the  colloidal  state,  or  in  other  words  the 
difference  between  true  and  colloidal  solutions,  can  be 
drawn  from  the  results  obtained  by  these  various  methods. 

As  regard  dialysis,  it  must  be  added  that  parchment 
paper  is  by  no  means  the  only  suitable  membrane  for 
this  purpose.  Collodion  membranes — left  by  the 
evaporation  of  a  solution  of  nitro-cellulose  in  ether- 
alcohol — and  fish-bladder  are  used  extensively,  especially 
in  physiological  work.  The  effect,  however,  is  always  the 
same  :  all  these  septa  permit  the  passage  of  true  solutions, 
i.e.,  of  liquids  in  which  the  dissolved  substance  is  present 
in  a  state  of  molecular  or  even  smaller  division.,  while 
they  retain  colloids.  If  we  ask  ourselves  for  the  reason 
of  this  phenomenon,  the  simplest  answer  is  obviously  that 
the  colloids  are  present  as  particles  or  aggregates  too 
large  to  pass  through  the  pores  in  the  membranes.  They 
may  of  course  actually  have  molecules  of  this  size  :  a 
dye-stuff,  Congo  Red,  e.g.,  which  lies  on  the  border 
between  true  and  colloidal  solutions,  contain  72  atoms, 
and  has  a  molecular  weight  of  654,  and  bodies  like  the 


12  ATTEMPTS  TO  DETERMINE  SIZE  OF  PARTICLES. 

albumins  no  doubt  have  molecules  of  still  more 
considerable  size.  On  the  other  hand,  this  explanation 
is  hardly  applicable  to  inorganic  sols,  especially  metal 
sols,  and  it  is  necessary  to  suppose  that  these  contain 
aggregates  formed  of  a  very  large  number  of  molecules. 

It  is,  however,  impossible  to  draw  any  conclusions  as 
to  absolute  size  from  the  process  of  dialysis,  in  view  of  its 
being  carried  out  without  any  pressure.  It  is  quite  eas}r 
to  retain  particles  by  ordinary  filtration  under  low 
pressure,  which  are  very  much  smaller  than  the  pores  in 
the  filtering  material  :  thus,  bacteria  are  retained  by  sand 
filters,  although  they  are  very  much  smaller  than  the 
interstices  between  the  sand  grains.  To  eliminate  this 
difficulty,  various  investigators  have  attempted  to  retain 
colloids  by  filtration  under  pressure  through  very  dense 
media,  such  as  the  filter  candles  used  in  bacteriological 
work.  Amongst  the  earliest  experiments  are  those  by 
Linder  and  Picton  with  arsenious  sulphide  sols.  These 
varied  according  to  the  method  of  preparation  :  most  of 
them  passed  through  the  filter  unaltered,  but  a  portion  of 
the  sulphide  from  certain  sols  was  retained.  For  these 
isolated  cases  a  limit  value  of  the  size  of  the  particles 
could  thus  be  obtained. 

Much  more  extensive  data  can  be  gathered  by  the 
aid  of  a  method  for  separating  colloids  invented  by 
H.  Bechhold,  and  called  by  him,  in  allusion  to  the 
ultra-microscope,  "  Ultra-Filtration."  Membranes  similar 
in  natures  to  those  used  in  dialysis  are  employed,  but 
these  are  not  immersed  in  the  solvent,  and  considerable 
pressure  is  used.  To  permit  of  the  latter,  the  membranes 
are  prepared  in  the  following  manner  :  strong  hard  filter- 
paper  is  impregnated,  generally  in  vacuo,  with  either  a 
gelatine  solution  or  acetic  acid  collodion  of  known 


ULTRA-FILTRATION.  1 8 

strength.  The  gelatine  filters  are  then  rendered  insoluble 
b5'  immersion  in  cold  formaldehyde  for  several  days,  while 
the  collodion  filters  are  immersed  in  water,  which 
gradually  dissolves  the  acetic  acid  and  leaves  a  gelatinous 
mass  of  nitrocellulose  in  the  substance  of  the  paper.  The 
filters  are  clamped  in  a  small  pressure  vessel  and  are 
supported  on  wire  gauze  and  perforated  metal,  so  that 
pressures  up  to  ten  atmospheres  can  be  used.  A  very 
important  feature,  predicted  and  subsequently  verified  by 
Bechhold,  is  the  ease  with  which  the  porosity  of  these 
ultra-filters  can  be  varied  by  altering  the  strength  of  the 
original  gelatine  or  collodion,  so  that  colloids  which  passed 
freely  through  a  "  2j  percent"  collodion  filter,  i.e.,  one 
made  from  a  collodion  containing  2^  per  cent  of  nitro- 
cellulose, could  be  entirely  retained  by  one  made  from  5 
per  cent,  collodion. 

The  size  of  the  pores  in  these  filters  can  be 
determined  by  two  methods  proposed  by  Bechhold  and 
by  a  third  one  suggested  by  the  author  and  also  tried  by 
Bechhold.  They  are  all  based  on  various  properties  of 
capillaries  and  need  not  be  discussed  here  in  detail. 
According  to  the  concentration  of  the  collodion  or  gelatine 
the  diameters  of  the  pores  lie  between  930  pp  and  21  pp. 
(The  pp,  which  is  the  unit  generally  employed  in  giving 
the  dimensions  of  such  particles  as  we  shall  have  to  deal 
with,  is  O'OOl/*,  while  the'/*,  which  is  generally  employed 
in  ordinary  microscopic  measurements,  is  0  001  mm.  The 
'pp  is  therefore  one  millionth  millimetre).  These 
dimensions  give  us  limits  for  the  sizes  of  the  particles 
retained  by  such  filters  :  if  the  particles  ar«  retained,  they 
are  probably,  though  not  necessarily,  larger  than  the 
pores ;  if  they  pass  through  the  filter,  it  is  reasonably 
certain  that  thejT  are  much  smaller  than  the  pores. 


I'l  RAYLEIGH'S  THEOREM. 

Evidence  tending  in  the  same  direction  and  towards 
the  same  limits  is  afforded  by  an  optical  property  of  the 
sols  to  which  reference  has  already  been  made  :  their 
turbid  appearance  in  the  Tyndall  cone.  If  such  a  cone  is 
produced,  e.g.,  in  a  gold  sol,  it  appears  greenish,  and  if 
this  greenish  reflected  light  is  examined  by  a  suitably 
placed  analyser,  it  is  found  to  be  polarized.  It  may  be 
worth  mentioning  that  herein  lies  its  difference  from  true 
fluorescence :  the  blue  light,  which  under  similar 
conditions  appears  in  a  solution  of  quinine  sulphate,  is  not 
polarized.  The  whole  phenomenon  has  been  mathe- 
matically investigated  by  Lord  Rayleigh,  and  it  has  been 
proved  that,  to  produce  it,  the  particles  reflecting  the 
light  must  be  small  compared  with  the  wave  length  of 
light.  The  values  of  the  latter  are  of  course  known 
accurately  and  lie  between  450  and  760  ^A{  f°r  the 
visible  spectrum.  These  limits  again  agree  very  well 
with  those  deduced  from  the  experiments  with  ultra-filters. 

At  the  same  time  they  answer  a  question  which  may 
already  have  occurred  to  the  reader  :  why  the  size  of 
such  particles  as  may  be  present  in  a  sol  cannot  be 
determined  directly  by  microscopic  observation  and 
measurement  ?  The  answer  is  that  particles,  which  are 
small  in  comparison  with  the  wave  length  of  light,  are 
invisible  in  the  ordinary  microscope.  It  was  shown  by 
Abbe  in  his  classical  investigations  en  the  microscope 
that  the  lower  limit  of  visibility  varied  from  0'8  to  0'2p, 
i.e.,  800  to  200  /*/*.  As  a  matter  of  experience,  all  sols 
which  appear  clear  to  the  eye,  and  many  which  appear 
distinctly  turbid,  entirely  fail  to  show  any  particles  with 
the  ordinary  methods  of  illumination. 

It  has,  however,  been  known  for  some  time  that  witli 
favourably  arranged  illumination  objects  of  sub- 


SLIT  ULTRA-MICROSCOPE. 


15 


microscopic  dimensions  could  be  rendered  visible.  Thus 
it  had  been  shown  by  Fizeau  and  Ambronn  that  slits  of 
much  smaller  width  than  the  lower  limit  of  visibility 
could  be  seen  if  they  were  strongly  illuminated  in  a  dark 
field.  This  observation  suggested  to  Zsigmondy  and 
Siedentopf  the  possibility  ot  rendering  visible  the 
individual  particles  which  collectively  produce  the  Tyndall 
phenomenon,  if  only  the  light  reflected  or  dispersed  by 
the  particles  was  permitted  to  enter  the  instrument,  but 
no  direct  rays  from  the  source  of  light. 


Fi<;.  3     "GENERAL  ARRANGEMENT  OK  THE  SLIT  ULTRA-MICROSCOPE." 

This  expectation  has  been  brilliantly  realised  and  the 
^Ultra-microscope" — as  the  microscope  fitted  with  this 
peculiar  form  of  illumination  has  been  called — has  now 
become  an  indispensable  instrument  of  research.  The 
principle  of  it  is  very  simple  :  a  powerful  beam  of  light  is 
thrown  horizontally  through  a  small  body  of  the  liquid, 
which  is  observed  through  a  microscope,,  the  axis  of  which 
is  vertical.  It  is  at  once  obvious  that  no  light  can  enter 
the  instrument,  except  such  as  has  bee"n  reflected  or 


SLIT    ULTRA-MICROSCOPE. 


dispersed   by  particles  present  in,  and  optically  different 

from,  the  liquid  itself,  i.e.,  they  must  either  be  opaque  or 

possess  a  refractive  index  different  from  that  of  the  latter. 

The  details  of  the  actual   arrangement,  which  have 


r-^^^rr----^^.: -._---  ..._:i'."t:r:.^£rr^.-v:XA .v_M  HUNGFJJiJENA.  .  :••' 

FIG.  4.— MICROSCOPE  WITH  QUARTZ  CHAMBER  FOR  ULTR  \-MICR:SCOPIC 
EXAMINATION  OK  LIQUIDS. 

called  for  the  exercise  of  very  great  ingenuity,  are  shown 
in  Fig.  3.  The  light  of  the  arc  lamp  d  is  projected  by 
the  lens  f  on  a  "  precision  "  slit  g,  the  width  and  height 


MEASUREMENTS  BY  SLIT  ULTRA-MICROSCOPE.  17 

of  which  can  be  adjusted  very  accurately.  An  image  of 
the  slit  is  formed  by  the  second  lens  h  and  projected  on 
the  illuminating  device  proper,  /,  which  is  substantially  a 
microscopic  objective.  This  throws  a  narrow  and  intense 
beam  of  light  through  a  cell  containing  the  liquid,  which 
is  seen  more  distinctly  in  Fig  4.  It  is  of  rectangular 
section  and  has  two  windows — the  one  in  front  admits 
the  beam  of  light,  while  the  second  one  is  at  the  top  and 
opposite  the  objective  of  the  microscope.  The  cell  is 
provided  with  a  funnel  at  one  end,  and  an  outlet  at  the 
other,  so  that  a  large  volume  of  liquid  can  be  passed 
through  the  cell  and  examined  in  one  setting. 

As  already  explained,  no  light  enters  the  microscope 
except  such  as  is  reflected  from  particles  in  the  liquid, 
and  the  image  formed  bears  no  relation  to  the  actual  size 
of  the  former,  but  depends  on  the  intensity  of  the 
illumination  and  the  qualities  of  the  microscope  only. 
Direct  measurement  of  the  particles  is  therefore  still 
impossible,  but  indirectly  the  diameters  may.,  with  certain 
assumptions,  be  determined  by  calculation  in  the  following 
manner  :  the  width  and  depth  of  the  illuminated  prism  of 
liquid  is  measured  by  means  ot  an  eye  piece  micrometer, 
and  the  number  of  particles  in  the  volume  thus  determined 
is  counted.  If  the  content  of  the  sol — say  the  amount  of 
gold  in  a  gold  sol — is  known,  the  weight  of  gold  in  the 
illuminated  volume  is  also  known,,  and  from  this  and  the 
number  of  particles  the  weight  of  a  particle  can  be  calcu- 
lated. Assuming  the  particles  to  be  a  simple  geometric 
shape — say  spherical  or  cubical — and  their  density  to  be 
that  of  solid  gold,  the  diameter  or  edge  can  again  be 
easily  calculated. 

In  this  manner  the  sizes  of  the  particles  in  many 
sols,  especially  of  the  metals,  have  been  calculated,  and 


18 


DARK  GROUND  CONDENSERS. 


these  agree  within  reasonable  limits  with  values  obtained 
by  Bechhold  from  experiments  with  ultra-filters. 

With  direct  sunlight,  particles  of  5  w  dia.  can  still 
be  distinguished.  In  many  sols,  however,  particularly 
organic  ones,  no  particles  can  be  seen  even  in  those  cir- 
cumstances, but  only  a  diffuse  light.  This  may  be  due 
either  to  their  small  size,  or  eKe  their  having  a  retractive 
index  very  near  that  of  the  liquid.  Particles  visible  in 
the  microscope  are  generally  described  as  microns  ;  those 
which  can  be  made  visible  by  the  ultra-microscope  as 
submicrons,  and  those  which  cannot  be  resolved  even  by 
the  latter  method  as  amicrons. 

The  apparatus  just  described  is  the  most  perfect  one 
for  the  observation  of  ultra-microscopic  particles,  and  the 
only  one  which  permits  measurements  to  be  made.  As  it 
is  rather  costly  and  requires  very  powerful  illumination,  a 
number  or  simpler  devices  have  been  introduced  in  which 
the  light  projected  through  the  film  of  liquid  is  parallel 
fc  f  with  the  axis  of  the  micro- 

scope, but  certain  arrange- 
ments are  made  to  prevent 
direct  light  from  entering 
the  instrument.  The  prin- 
ciple of  all  these  appliances 
is  the  same,  and  will  be 
readily  understood  by  refer- 
ence to  Fig.  5,  which 
shows  a  section  of  the 
"paraboloid  condenser"  made 
by  Carl  Zeiss.  The  condenser  is  a  paraboloid  of  revolution, 
of  glass  or  rock  crystal,  bounded  by  two  parallel  planes. 
Parallel  rays  entering  the  condenser  are,  as  is  well  known, 
reflected  into  the  focus  of  the  parabola,  and  the  top  of  the 


FIG.  5.— SECTION-  OK  PARABOLOID 
CONDENSER.  SHOWING  VATH  OK  KAYS. 


SOLS  AS  SYSTEMS  OF  TWO  PHASES.  19 

condenser  is  so  adjusted  that  this  focus  falls  on  the  surface 
of  the  slide,  which  is  of  a  definite  thickness.  A  central 
stop  covers  the  bottom  of  the  paraboloid,  and  permits  only 
such  rays  to  pass  as  will,  after  reflection,  strike  the  surface 
under  an  angle  greater  than  the  critical  angle,  so  that 
they  would  be  totally  reflected  by  the  top  of  the  condenser, 
if  in  contact  with  air.  By  placing  water  or  cedar  oil 
between  this  surface  and  the  slide  the  light  is  enabled  to 
pass  through  the  slide  and  any  liquid  placed  on  it,  but  is 
totally  reflected  at  the  cover  glass  placed  oil  the  liquid. 
If,  however,  the  latter  contains  particles,  light  is  dispersed 
from  them — as  indicated  by  the  clotted  lines — which  can 
enter  the  microscope  and  form  an  image. 

This  and  similar  condensers  can  be  fitted  to  any 
microscope,  and  can  be  used  for  rendering  visible  sub- 
microns,  a  Nernst  lamp  or  incandescent  gas  mantle  being 
quite  sufficient  for  illumination.  Measurements  are  not 
possible,  as  the  volume  of  liquid  cannot  be  accurately 
determined. 

From  the  foregoing  it  appears  that  colloidal  solutions 
in  many  cases  contain  particles  which  can  be  made  visible 
by  appropriate  means,  and  whose  size  can  be  determined. 
The  sizes  are  still  very  considerably  larger  than  the  limit 
values  for  the  size  of  various  molecules  arrived  at  by 
numerous  investigators  employing  a  great  variety  of 
methods. 

In  this  presence  of  particles  of  sizes  greatly  exceeding 
molecular  dimensions,  must  be  sought  one  of  the  funda- 
mental differences  between  colloidal  and  true  solution. 
To  emphasise  this,  we  shall  in  future  employ  the  termin- 
ology now  in  general  use,  by  calling  the  colloidally 
dissolved  substance  the  "disperse"  and  the  solvent  the 
"  continuous  "  phase,  terms  which  explain  themselves  and 


20  GROUNDS  OF  CLASSIFICATION. 

form    a    constant    reminder    of    the     fact    that    colloidal 
solutions  are  systems  of  two  phases. 

A  knowledge  of  these  sizes,  however,  does  not,  by 
itself  explain  the  striking  differences  between  various 
types  of  colloids,  nor  does  it  form  a  basis  of  classification. 
These  differences  are  particularly  marked  as  regards  the 
behaviour  towards  electrolytes,  which  precipitate  one  type 
— the  inorganic  sols — even  in  small  quantities,  while  they 
have  little  effect  on  many  organic  colloids — such  as 
gelatine,  agar,  etc. — even  in  large  quantities.  A  division 
into  lyophobic "  and  lyophilic "  colloids,  i.e.,  such  as 
remain  reluctantly  and  such  as  remain  freely  in  solution, 
has  been  based  on  this  difference  by  Perrin  and  Freundlich. 
These  names  are,  however,  merely  descriptive,  and  the 
classification  adopted  by  Wolfgang  Ostwald  appears  for 
many  reasons  preferable.  This  takes  for  its  basis  the 
consideration  that  the  disperse  phase  of  a  sol  may  consist 
of  either  liquid  or  solid  particles,  and  explains  very  many 
of  the  differences  between  the  two  principal  types  of 
colloids  on  this  basis.  It  is  only  natural  that  a  variety  of 
factors  are  necessaiy  to  make  the  reasoning  which  underlies 
this  classification  convincing.  Some  of  these  will  be  con- 
sidered later  on,  and  for  the  moment  we  will  confine 
ourselves  to  studying  in  some  detail  one  physical  property 
which,  even  if  taken  by  itself,  makes  some  such  assumption 
as  that  quoted  above  necessary,  that  is  the  viscosity  of 
various  types  of  sols. 


CHAPTER  III. 

THE  reader  is  no  doubt  aware  of  what  is  understood, 
in  a  general  way.,  by  the  viscosity  of  a  liquid :  the 
resistance  offered  to  shearing,  stirring  or  the  flow  through 
a  capillary  tube.  If  a  liquid  is  contained  between  two 
parallel  plates  and  one  of  these  is  moved  with  a  constant 
velocity  in  its  own  plane,  a  certain  force  is  required, 
which  depends  on  the  velocity,  the  surface  and  distance 
of  the  two  plates,  and  of  course  on  the  nature  and 
temperature  of  the  liquid.  This  gives  us  the  definition 
of  the  viscosity  coefficient"  at  any  temperature:  the 
force  required  to  move  a  plate  of  unit  surface  separated 
from  another  plate  of  the  same  size  by  a  layer  of  liquid 
of  unit  thickness,  at  unit  velocity.  Such  coefficients 
for  many  liquids,  expressed  in  gramme-centimetre-second 
units,  can  be  found  in  Landolt  and  Bornstein's  tables. 
They  ail  decrease  rapidly  in  the  rising  temperature. 

As  regards  colloidal  solutions,  they  divide  themselves 
into  two  classes,  if  the  increase  of  viscosity,  compared 
with  that  of  the  solvent  or  continuous  phase,  is  taken  as 
the  basis  of  classification.  One  class,  the  metal  and 
sulphide  sols  in  particular,  shows  a  viscosity  only  very 
slightly  higher  than  that  of  water.  The  other,  which 
comprises  principally  the  organic  colloids,  such  as  albumin, 
gelatine,  gum  arabic,  agar,  etc.,  shows  a  very  marked 
increase  of  viscosity,  even  if  the  percentage  of  dissolved 
matter  is  small.  To  explain  this  difference  some  of  the 
earlier  investigators  in  this  field,  more  especially  Quincke, 
resorted  to  reasoning  by  analogy,  based  on  the  known 
behaviour  of  systems  containing  the  disperse  phase  in  a 


22  VISCOSITY  OF    SUSPENSIONS. 

much  coarser  form  than  do  the  sols.  It  is  a  fact  familiar 
to  everybody  who  has  stirred  up  finely  divided  solid 
matter,  such  as  a  precipitate  of  calcium  carbonate  or 
barium  sulphate,  with  water,  that  even  20  or  30  per  cent 
do  not  offer  a  very  great  resistance  to  stirring,  i.e.,  do  not 
cause  a  great  increase  of  viscosity.  On  the  other  hand, 
it  is  equally  well  known  that  systems  of  two  liquids 
insoluble  in  each  other,  generally  called  emulsions,  can 
under  certain  circumstances  show  enormous  increase  in 
viscosity  compared  with  either  phase.  Various  phar- 
maceutical preparations  coming  under  this  head  are 
familiar  to  everyone  ;  extreme  cases  are  represented  by 
some  emulsions  used  as  lubricants,  and  by  Pickering's 
emulsions  with  99  per  cent  of  oil  in  1  per  cent  of  soap 
solution,  wrhich  could  be  cut  into  cubes. 

Applying  these  considerations  to  sols,  we  are  led  to 
the  conclusion — now  generally  accepted — that  in  those 
which  show  a  low  viscosity  the  disperse  phase  is  present 
as  solid  particles,  while  in  the  sols  with  high  viscosity  the 
disperse  phase  is  liquid.  An  albumin  sol,  for  instance, 
would  consist  of  a  dilute  solution  of  albumin,  in  which  are 
dispersed  drops  or  globules  consisting  of  a  much  more 
concentrated  solution.  This  conception  is  certainly  not 
an  easy  one,  and  will  receive  further  discussion  when  we 
arrive  at  the  detailed  consideration  of  the  class  of  colloids 
to  which  it  applies,  but  it  is  the  only  one  which  explains 
all  their  peculiarities.  Nor  does  it  rest  only  on  the 
analogies  set  forth,  but  is  strongly  supported  by 
mathematical  investigations,  undertaken  independently 
and  by  entirely  different  methods,  by  A.  Einstein  and  by 
the  author  during  recent  years.  These  show,  that  the 
presence  of  solid  particles  in  a  liquid  can  only  raise  the 
viscosity  by  small  amounts  simply  proportional  to  the 


OSWALD  S  CLASSIFICATION  J  STOKES    FORMULA.  23 

volume  of  solid  matter  present ;  when  a  few  per  cent,  of 
— apparently — solid  matter  like  agar  raise  the  viscosity 
some  hundred  times,  it  follows  that  the  disperse  phase 
cannot  be  solid. 

Systems  consisting  of  solid  particles  ot  microscopic 
size  distributed  through  a  liquid  are  generally  called 
suspensions,  while  those  having  two  liquid  phases  are 
called  emulsions.  Wolfgang  Ostwald  accordingly  calls 
the  sols  resembling  the  former  Suspensoids "  and  the 
sols  which  resemble  the  latter,  in  showing  markedly  high 
viscosities,  "Emulsoids,"  which  terminology  we  shall  adopt 
in  future.  These  two  classes  coincide  to  a  very  great 
extent  wtth  the  lyophobic  "  and  lyophilic  "  colloids. 

The  suspensoids  show  a  much  more  uniform 
behaviour  towards  various  influences  than  the  emulsoids, 
and  will,  therefore,  be  considered  first.  We  are  now  in 
the  possession  of  two  distinct  data  :  we  kiiowr  the 
approximate  size  of  the  particles  in  a  large  number  of 
sols,  and  we  have  concluded  that  they  are  solid.  The 
next  step  is  to  co-ordinate  these  two  factors  by  examining 
the  behaviour  of  small  particles  suspended  in  a  liquid,  and 
especially  how  this  changes  when  their  size  decreases  to 
ultra-microscopic  dimensions. 

It  was  shown  by  Stokes  in  1850  that  a  small  sphere 
falling  in  a  liquid  soon  assumes  a  constant  velocity,  which 
is  given  by  a  formula  that  has  been  used  in  an  enormous 
number  of  most  important  investigations,  and  has  led  to 
numerous  results  of  great  value.  If  we  call 

r  the  radius  of  the  particle, 
s  the  specific  gravity  of  the  same, 
s'  the  specific  gravity  of  the  liquid, 
?7  the  viscosity  coefficient  of  the  latter, 
g  the  gravity  constant, 


24  SECRETIONS  FROM  STOKES*   FORMULA. 

the  constant  velocity  of  the  particle  is  : — 

v==2r>  (s-s')g. 

9'? 

It  is  obvious  that  the  difference  (s — s )  may  be 
positive,  zero  or  negative,  that  is  the  particle  may  sink, 
remain  stationary  or  rise  if  its  specific  gravity  is  greater, 
equal  to,  or  smaller  than  that  of  the  liquid.  It  is  also 
obvious  that  the  velocity,  in  whichever  direction,  is 
inversely  proportional  to  the  viscosity  of  the  liquid  :  a 
particle  of  a  given  size  and  weight  will  sink  several 
hundred  times  faster  in  water  than  in  castor  oil.  The 
point  which,  in  the  present  connection,  interests  us  most 
is,  that  the  velocity  is — other  things  being  equal — 
proportional  to  the  square  of  the  radius. 

To  fix  ideas  it  will  be  useful  to  consider  an  example 
in  figures,  say  a  gold  particle  of  1  /^  radius  or  2  f 
diameter.  Introducing  the  proper  values  (all  in  cm.,  gr. 
and  seconds),  viz.,  r  =  10~4,  s  =  19'3,  s'  (water)  =  1,  g 
=980,  r\  at  20°  =  O'Ol,  we  find  the  velocity  of  the 
particle  about  0.04  mm.  per  second,  or  2.4  mm.  per  minute. 

This  is,  of  course,  a  considerable  speed,  and  means 
in  other  words,  that  such  a  suspension  of  gold  particles 
would  clear  at  the  rate  of  2.4  mm.  per  min.  from  the  top, 
and  would  be  clear  to  a  depth  of  about  14  cm.  after  one 
hour. 

Assuming  now  the  radius  to  be  1/100  of  that  just 
considered,  or  10  /-*/•*,  which  is  the  size  of  the  particles  in 
a  red  gold  sol  as  determined  by  many  measurements,  the 
velocity  would  be  1/10,000  of  that  calculated.  This 
makes  it  only  00144  mm.  per  hour,  or  about  10  mm.  in 
one  month.  With  particles  of  lower  specific  gravity  the 
rate  of  settling  \vould,  of  course,  be  still  lower,  e.g.,  with 


BROVVNIAN   MOVKMKNT.  25 

a  sp.  gr.  of  3  it  would  be  a  little  over  (me  ;//;//.  per 
month. 

This  little  calculation  shows  us,  that  with  particles 
sufficiently  small  and  not  much  heavier  than  the  liquid, 
a  suspension  may  appear  very  stable  and  take  a  very  long 
time  to  show  an}r  marked  clearing.  At  the  same  time, 
we  already  are  familar  with  one  property  of  sols — their 
liability  to  undergo  irreversible  transformation — which 
precludes  our  accepting  mere  smallness  of  particles  as  a 
sufficient  explanation  of  the  stability  of  many  sols.  A 
suspension  is  reversible  :  when  it  has  settled,  however 
long  the  process  may  take,  it  can  be  shaken  up  again,  and 
this  may  be  repeated  indefinitely.  This  is  not  possible 
with  a  sol  like  the  metalic  sols  ;  when  this  has  coagulated, 
the  gel  cannot  by  mere  shaking  be  transformed  back  into 
the  original  sol.  We  are  therefore  forced  to  the 
conclusion  that  the  particles  are  subject  to  other  influences 
besides  those  of  gravity  and  viscosity,  Investigation  has 
shown  this  reasoning  to  be  correct,  insomuch  as  the 
particles  are  in  violent  motion,  and  are  also  electrically 
charged. 

The  motion  of  the  particles  in  a  sol  is  the  most 
striking  feature  in  the  ultra-microscopic  image.  It  is, 
however,  visible  even  with  much  larger  particles  and 
ordinary  illumination,  and  was  first  observed  as  far  back  as 
1827  by  Dr.  Brown,  a  botanist,  after  whom  it  is  called  the 
Brownian  movement. 

The  movement  is  composed  of  an  oscillating  motion 
of  the  particles  round  a  central  position,  and  an  erratic 
translatoiy  motion.  Description  is  rather  inadequate,  but 
the  movement  can  be  seen  very  well  in  a  suspension  of 
gamboge  (the  ordinary  water  colour)  with  magnifications 
of  about  500  diameters  and  such  simple  dark  ground 


26  BROWMAN    MOVEMENT. 

illumination  as  can  be  obtained  with  a  central  stop  in  the 
ordinary  condenser. 

It  was  soon  found  that  the  motion  decreased  as  the 
particles  grew  larger,  and  did  not  show  itself  at  all  if  the 
diameter  exceeded  a  certain  size.  Various  suggested 
causes,  such  as  vibration,  convection  currents  due  to 
changes  of  temperature  or  of  concentration  were  gradually 
eliminated  by  experiment,  and  it  became  evident  that  the 
movement  could  hardly  be  due  to  any  transient  cause 
when  it  was  found  that  small  gas  bubbles  in  liquid, 
enclosed  in  rock  crystal,  still  showed  it.  The  opinion 
gained  ground  that  the  origin  of  the  movement  had  to  be 
looked  for  in  some  factor  inherent  in  the  liquid  state. 
The  study  of  the  phenomenon  received  an  enormous 
impetus  by  the  invention  of  the  ultra-microscope,  as  the 
very  small  particles  disclosed  by  it  for  the  first  time 
showed  it  so  vividly  that  Zsigmondy  was  inclined  to  look 
on  it  as  something  differing  not  only  in  degree  but  also  in 
kind  from  the  Brownian  movement  as  known  up  to  then. 
Methods  were  devised  which  facilitated  the  measure- 
ment of  the  amplitude  of  oscillation  :  Svedberg  allowed 
the  liquid  to  flow  through  the  field,  while  Siedentopf, 
photographed  it  on  a  falling  plate.  In  both  cases  the 
oscillating  motion  of  the  particle  is  combined  witli  the 
rectilinear  motion  of  the  liquid  or  the  plate,  so  that  the 
images  seen  directly  in  the  first,  and  photographed  in  the 
second  method,  appear  as  wave  lines,  which  permit 
convenient  measurement  of  the  amplitude  and  of  the 
period,  i.e.,  the  time  between  two  similar  positions. 
Svedberg  found  the  first  two  mathematical  relations 
between  these  quantities  :  the  amplitude  is  directly 
proportional  (for  particles  of  one  size)  to  the  period,  and 
inversely  proportional  to  the  viscosity  of  the  liquid. 


THIORV   OF   BROWMAN    MOVEMENT.  "2~ 

In  1906  the  phenomenon  was  treated  mathematically, 
by  entirely  different  methods,  by  Einstein  and  by  v. 
Smoluchowski,  on  the  definite  assumption  that  it  wax 
due  to  the  impacts  of  the  molecules  of  the  liquid  on  the 
particle.  The  two  physicists  arrived  at  formulae  identical 
except  for  a  numerical  constant,  and  showing  that  the 
motion  was  determined  only  by  two  physical  constants  ot 
the  liquid,  i.e..  temperature  and  vicosity,  and  by  the 
diameter  of  the  particles,  but  was  indipendent  of  the 
maxs  of  the  latter.  They  also  confirm  Svedberg's  results 
a^  regard-  amplitude  and  period.  In  a  further  investiga- 
tion, involving  experimental  methods  of  extreme 
ingenuity.  Perrin  showed  that  the  whole  phenomenon 
conformed  to  conclusions  drawn  from  the  kinetic  theory, 
and  that  there  was  no  essential  difference  between  these 
particlex  and  molecules  ;  that,  in  other  words,  they  could 
be  treated  as  molecules  of  a  substance  with  extremely 
high  molecular  weight  :  of  the  order  of  1000  millions  in 
the  case  of  the  mastic  particles  used  by  him.  Perrin's 
paper  has  been  translated  into  English  by  Soddy  and  the 
reader  is  referred  to  this  work  for  full  details. 

It  follows  from  Perrin's  investigation  that  particles  in 
Brownian  movement — like  molecules  of  a  gas  or  a 
disolved  substance — tend  to  fill  the  space  in  which  they 
are  contained  according  to  definite  laws.  The  movement 
must  therefore  be  considered  as  one  of  the  factors  which 
keep  a  sol  stable,  but  is  not,  by  itself,  sufficient  to  account 
for  its  stability,  as  particles  showing  moderate  Brownian 
movement  may  still  settle  with  comparative  rapidity. 
The  stability  is  intimately  connected  with  the  electric 
charge,  to  which  reference  has  already  been  made.  It 
may  be  said  generally  that  any  substance  in  contact  with 
water  and  many  other  liquids,  assumes  an  electric  charge, 


28  ELECTRIC    CHARGE    ON     DISPERSE    PHASE. 

the  origin  of  which  is  not  definitely  explained.  Most 
substances  become  negatively  charged  in  contact  with 
water  :  the  charge  can  be  varied  and  even  reversed  by 
the  addition  of  electrolytes.,  and  may  become  zero  at 
suitable  concentrations.  In  this  condition,  as  shown  by 
Burton  and  by  Hardy,  sols  are  particularly  unstable,  and 
tend  to  precipitate. 

It  need  hardly  be  mentioned  that  the  electric  charge 
is  not  confined  to  submicroscopic  particles,  but  is  found 
equally  on  the  particles  of  a  course  suspension.  It  has 
also  been  known  for  a  considerable  time  that  the  speed 
of  settling  in  many  suspensions — which  settle  in  any 
event — can  be  increased  by  the  addition  of  electrolytes. 
The  greater  sensitiveness  of  the  highly  dispersed  systems 
must  be  ascribed  to  the  very  much  greater  charge  due  to 
the  enormous  increase  in  surface.  At  the  same  time, 
while  the  existence  and  the  stabilising  influence  of  the 
charge  is  fully  established,  it  must  be  said  that  the  origin 
of  the  charge  and  the  mechanism  of  its  action  is  still 
rather  obscure. 

Before  we  proceed  to  study  stispensoids  in  detail,  it 
may  be  worth  mentioning  how  the  electric  charge  on  the 
particles  can  be  demonstrated.  This  can  be  done,  for 
instance,  by  placing  the  sol  to  be  examined  into  the  bend 
of  a  U-tube  (Fig.  6)  and  filling  the  limbs  with  pure  water, 
into  which  dip  electrodes  connected  with  some  source  of 
current.  The  particles  gradually  wander  into  the  water 
surrounding  the  pole  of  opposite  sign,  so  that  negatively 
charged  particles  wander  to  the  anode. 

Another  very  convenient  method,  permitting  the  use 
of  small  volumes  of  liquid  is  the  microscopic  method,  as 
used  first  by  Cotton  and  Mouton.  An  ordinary  microscopic 
slide  (Fig.  7)  is  provided  with  a  pair  of  electrodes  of 


METHODS  OF  DEMONSTRATING  ELECTRIC  CHARGE. 


platinum  foil,  which  are  connected  by  suitable  leads  to  a 
couple  of  cells  or  accumulators.  A  drop  of  the  liquid 
under  examination  is  placed  on  the  slide  so  as  to  touch 


Fic.6. 


both  electrodes  and  covered  with  a  cover  glass.  The 
preparation  is  examined  under  the  microscope — using  one 
of  the  ultra-condensers  described  in  the  previous  chapter, 


AMOUNT    OF    CHARGE    ON    PARTICLE 

if  necessary — and  the  particles  are  seen  to  travel  to  the 
anode,  if  negatively,  or  the  cathode,  if  positively  charged. 
Both  the  U-tube  and  the  microscopic  method  also 
permit  the  actual  charge  on  a  particle  to  be  measured.,  by 
observing  the  rate  of  travel  and  the  strength  of  the  electric 
field,  i.e.,  the  difference  in  voltage  divided  by  the  distance 
of  the  electrodes.  We  need  only  mention  here  that  a 
very  large  number  of  different  materials  have  been  so 
examined,  and  that  the  charge  carried  by  a  particle  is 
very  much  the  same  in  all  cases. 


CHAPTER  IV. 

IN  the  preceding  chapter  we  have  fully  characterised 
the  suspensoids  are  systems  containing  the  disperse  phase 
as  solid  particles  below  a  certain  size,  in  constant 
movement  and  electrically  charged.  A  few  typical 
suspensoids — the  gold,  silver,  antimony  and  arsenic 
sulphide  sols — have  already  been  referred  to  in  the  first 
chapter,  where  directions  are  given  for  preparing  them. 

These  directions  have  one  feature  in  common  :  the 
metal  is  reduced  or  the  sulphide  produced  by  a  suitable 
reaction  in  very  dilute  solution.  This  procedure  is 
applicable  in  many  cases,  though  not  universally,  and  sols 
of  silver,  copper,  mercury,  platinum,  palladium,  etc.,  have 
beeri  prepared  by  reducing  very  dilute  solutions  of  their 
salts  with  a  great  variety  of  reducing  agents,  such  as 
•hydrogen,  hydrazine,  and  hydroxylamine  compounds, 
acrolein,  hypophosphorous  acid,  and  many  others.  Many 
sulphides  and  haloid  compounds  can  also  be  obtained  in  a 
similar  fashion.  The  reason  why  all  these  processes  can 
be  carried  out  in  very  dilute  solutions  only  is  clear :  we 
already  know  that  the  suspensoid  sols  are  precipitated  by 
varying,  but  always  small,  amounts  of  electrolytes.  As 
practically  all  the  reactions  mentioned  lead  to  the 
formation,  not  only  of  the  disperse  phase  which  it  is 
desired  to  obtain,  but  also  of  electrolytes,  it  is  obvious 
that  the  concentration  of  these  must  be  kept  below  a 
certain  limit,  or,  in  other  words,  that  it  is  necessary  to 


32  METHODS    FOR    PREPAKIXG    SUSPENSOIDS. 

work  with  very  dilute  solutions.  Many  sols  can  be  con- 
centrated considerably  after  removal  of  the  electrolyte 
by  dialysis. 

A  number  of  other  methods  have  been  applied  with 
considerable  success,  several  of  which  may  be  classed 
together  as  "  disintegration  methods."  The  most 
important  of  these  is  that  of  Bredig,  first  published  in 
1898,  which  consists  in  producing  a  small  electric  arc- 
between  electrodes,  made  of  the  metal  to  be  dispersed, 
under  water.  With  suitably  proportioned  electrodes, 
current  strength,  and  cooled  liquid,  sols  of  many  metals 
can  be  prepared,  including  platinum,  iridium,  palladium, 
gold,  silver,  copper,  lead  and  others.  The  method  has 
been  considerably  developed  and  improved  by  Svedberg, 
who  substituted  for  the  arc  an  oscillating  discharge 
between  electrodes  of  aluminium  or  zinc  (which  resist 
disruption)  between  which  the  metal  to  be  dispersed  is 
suspended  as  foil.  He  also  used,  instead  of  water,  a 
number  of  organic  compounds,  such  as  ether,  pentane. 
isobutylalcohol,  etc.,  and,  by  cooling  with  liquid  air,  was 
able  to  prepare  sols  of  the  alkali  metals,  barium, 
strontium,  and  many  others.  An  interesting  feature  of 
some  of  these  sols  is  their  colour,  which  in  most  cases  is 
the  same  as  that  of  the  vapour. 

Another  disintegration  method,  invented  by  Kuzel, 
and  applied  particularly  to  the  metals  of  the  bismuth  and 
chromium  groups,  consists  in  grinding  the  material  as 
fine  as  possible  in  ball  mills  and  then  treating  it  repeated- 
ly and  for  many  days  alternately  with  strong  alkali  and 
acid.  When  these  are  finalty  replaced  by  water,  the 
metal  is  found  to  be  reduced  to  so  fine  a  state  that  it 
forms  a  sol. 

In  a  few  cases  no  treatment  at  all  is  required,  but 


METHODS    OF    PREPARING    SUSPENSOIDS    (continued)          33 

the  metal  goes  directly  into  colloidal  solution  on  contact 
with  water,  or  at  leact  on  boiling.  Lead,  e.g.,  has  been 
shown  by  Mine.  Traube-Mengarini  and  A.  Scala  to  form 
a  sol  at  once  011  contact  with  distilled  water  :  if,  however, 
no  precautions  are  taken  to  exclude  oxygen,  the  dissolved 
lead  is  promptly  transformed  into  hydroxide.  The  same 
investigators  showed  that  a  silver  sol  could  also  be 
obtained  by  boiling  distilled  water  for  some  time  in  silver 
vessels ;  that  copper  sols  are  formed  under  the  same 
conditions,  has  been  known  for  some  time. 

A  third  procedure  is  that  called  by  Graham 
*  peptisation,"  in  allusion  to  the  transformation  of  insoluble 
into  soluble  compounds  by  digestion.  An  example  will 
best  illustrate  the  characteristic  feature  of  this  method, 
for  instance,  the  production  of  a  cadmium  sulphide  sol. 
The  sulphide  is  precipitated  from  an  ammoniacal  solution 
of  cadmium  sulphate,  washed  and  finally  suspended  in 
distilled  water.  If  hydrogen  sulphide  is  now  passed 
through  the  water,  the  suspension  gradually  becomes 
milky  and  finally  perfectly  clear  golden  yellow  with  a 
slight  red  surface  colour.  The  dissolved  gas  may  be 
displaced  by  nitrogen,  or  removed  by  boiling,  the  sol 
being  extremely  stable. 

Of  all  the  striking  properties  of  the  suspensoid  sols 
their  instability  in  the  presence  of  electrolytes  is 
probably  the  most  striking,  and  has  received  a  very  large 
amount  of  attention  and  study,  particularly  from  English 
investigators — Linder  and  Picton,  Hardy,  Burton,  and 
others.  To  summarise  their  results,  at  the  same  time 
excluding  matter  which  goes  much  beyond  the  scope  of 
this  work,  and  is  in  part  still  highly  controversial,  it  may 
be  said  that  the  first  step  in  the  coagulation  of  suspensoids 
?s  the  neutralisation  of  their  electric  charges  by  that  of 

2 


34         ELECTROLYTE  COAGULATION.        EFFECT  OF  VALENCY. 

the  opposite!}^  charged  ions  of  the  electrolyte.  As  most 
suspensoids  are  negatively  charged,  the  active  ion  is 
accordingly  the  positive,  or  cation,  of  the  electrolyte. 
Each  electrolyte  has  to  be  present  in  a  definite  con- 
centration to  produce  coagulation,,  and  a  veiy  large 
number  of  investigations  have  been  directed  to 
ascertaining  these  minimum  concentrations  for  various 
sols,  and  a  large  range  of  electrolytes.  The  subjoined 
table  gives  the  results  of  one  of  the  most  extensive  series 
by  Freundlich.  The  sol  wras  an  As2  83  sol,  containing 
7.539  millimoles  (i.e.,  1'854  grammes)  of  the  negatively 
charged  sulphide  per  litre  ;  the  electrolyte  concentrations 
are  given  in  millimoles  per  litre  : — 

Al  C13          0.093 
1 3  0.095 


K  Cl 

49.5 

MgCl2 

0.717 

KN03 

50.0 

Ca  C12 

0.649 

NaCl 

51.0 

Ba  C12 

0.691 

LiCl 

58.4 

Ba(N03)3 

0.687 

HC1 

30.8 

The  first  column  contains  the  electrolytes  with 
monovalent,  the  second  those  with  divalent,  and  the  third 
those  with  trivalent  cations.  It  will  at  once  be 
noticed  that — in  each  column — the  quantity  of 
electrolyte  depends  only  on  the  cation  :  practi- 
cally the  same  molar  concentration  of  potassium 
chloride  and  nitrate,  barium  chloride  and  nitrate,  and 
aluminimum  chloride  and  nitrate  produces  the  same  effect^ 
which  therefore  in  each  case  depends  only  on  the  amount 
of  K*,  Ba'  '  and  Al"  present.  A  very  striking  difference 
however,  shows  itself  between  salts  containing  cations  of 
different  valency,  that  is  between  the  three  columns  of 
the  table.  The  concentration  of  the  monovalent  cations 
is  roughly  70-80  times  greater  than  that  of  the  divalent 


EFFECT  OF  \ 


ALENCV  (continued).  35 


and  600  times  greater  than  tljat  of  the  trivalent  cation. 
This  phenomenon,  which  Hardy  was  the  first  to  study,  is 
probably    best  explained    by   the  theory   propounded  by . 
Freundlich,  to  which  wre  shall  have  occasion  to  refer  when 
considering  adsorption. 

An  equally  important  point  in  this  connection — the 
comparative  behaviour  of  ions  of  different  valency — was 
first  investigated  by  Linder  and  Picton.  When  the 
electrolyte  is  added  to  the  sol,  the  particles  are  discharged 
and,  as  they  no  longer  repel  one  another,  are  free  to 
approach  and  to  form  larger  aggregates,  which  settle  more 
or  less  rapidly.  These  precipitates  always  contain  some 
of  the  precipitating  cation,  and  investigation  show's  that 
the  amounts  of  different  cations  found  are  equivalent. 
This^  fact — which  is  somewhat  surprising  in  view  of  the 
great  difference  in  concentration — becomes  intelligible 
at  once  when  we  remember  that  equivalent  quantities  of 
all  ions  carry  the  same  charge,  and  are  therefore  able  to 
neutralize  the  same  amount  of  oppositely  charged 
suspensoid  particles. 

We  have  so  far  referred  exclusively  to  sols  containing 
only  the  disperse  phase  and  the  small  amount  of 
electrolytes  produced  by  the  reaction  which  gives  rise  to 
the  formation  of  the  disperse  phase.  Many  reactions, 
however,  for  instance,  that  between  lead  compounds  and 
chromates,  or  barium  salts  and  sulphates,  never  lead  to 
sol  formation,  even  if  carried  out  in  very  great  dilution  in 
pure  aqueous  solutions.  In  other  cases  such  reactions 
do  produce  sols,  which,  however,  are  very  unstable.  It 
has  been  known  for  some  time  that  the  addition  of  very 
small  amounts  of  colloids  belonging  to  the  second  or 
"emulsoid  "  groups  greatly  increased  the  stability  of  sols  : 
Fayaday  observed,  that  the  gold  sol  which  he  obtained  by 


86  PROTECTIVE  COLLOIDS. 

reducing  gold  chloride  solution  with  a  solution  of 
phosphorus  in  ether  could  be  kept  unaltered  much  longer 
if  a  little  jelly"  was  added  to  it.  Similarly,  if  the 
colloid  is  added  beforehand  to  one  of  the  reacting 
solutions,  insoluble  precipitates  can  be  obtained  in  so  fine 
a  state  of  dispersion  that  they  form  sols  :  thus  a  small 
addition  of  casein  makes  it  possible  to  obtain  sols  of  lead 
chromate,  barium  sulphate,  etc.  The  sols  so  obtained 
also  require  very  much  larger  amounts  of  electroytes  for 
precipitation  than  do  suspensoids  without  such  additions, 
which  are  accordingly  in  this  connection  described  as 
protective  colloids."  They  all  belong  to  the  emulsoids, 
of  which  we  already  know  that  they  are  much  less 
sensitive  to  electrolytes  than  the  suspensoids,  and  their 
action  in  protecting  the  latter  is  probably  best  explained 
by  Bechhold's  assumption  that  each  particle  of  the 
suspensoid  surrounds  itself  with  a  layer  of  the  emulsoid 
and  then  possesses  the  electrical  properties  of  the  latter. 

Although  many  readily  available  substances,  such  as 
gelatine,  albumin,  casein,  isinglass,  etc.,  can  be  used  as 
protective  colloids,  one  group  of  compounds  has  acquired 
great  prominence  during  the  last  few  years.  These  are 
certain  products  of  the  hydrolysis  of  albumin  by  alkalies, 
first  described  by  Paal  and  his  collaborators  as  protalbic  " 
and  tysalbic "  acids,  and  used  by  them  in  the 
preparation  of  a  very  large  number  of  different  metal  sols^ 
These  show  very  great  stability,  and  the  silver  and 
mercury  sols,  for  instance,  can  be  evaporated  to  dryiiess 
and  then  readily  re-dissolved  in  water. 

The  protective"  effect  of  different  emulsoids  varies 
between  very  wide  limits,  and  may  perhaps  offer  one  of 
the  most  delicate  means  for  their  differentiation.  It  has 
been  studied  systematically  by  Zsigmondy,  who  deter- 


SYSTEMS     OF    TWO    LIQUID     PHASES.  37 

mined  the  quantity  of  colloid  just  necessary  to  protect  a 
definite  volume  of  a  standard  gold  sol  from  coagulation  by 
-a  given  quantity  of  sodium  chloride.  The  relative  figures 
vary  from  O'OOS  for  gelatine,  0'02  for  isinglass,  and  0'2  for 
egg  albumin,  to  12  for  dextrine  and  25  for  potato  starch. 
They  are  usually  referred  to  as  the  "gold  values"  or 
gold  figures  "  of  the  colloids. 


CHAPTER    V. 

IN  the  last  two  chapters  we  have  considered  systems 
in  which  the  disperse  phase — whether  of  microscopic  or 
ultra-microscopic  dimensions — consists  of  solid  or,  more 
correctly,  rigid  particles,  and  we  now  proceed  to  the 
study  of  systems  of  two  liquid  phases.  These  present 
several  very  striking  peculiarities  which  can  only  be 
properly  explained  by  observing  either  natural  or  artificial 
"  emulsions/'  i.e.,  fairly  coarse  dispersions  of  one  liquid  in 
another,  with  which  it  is  not  miscible.  Roth  natural  and 
artificial  systems  of  this  kind  are  numerous  and  important : 
of  the  former  we  may  mention  milk,  which  is  an  emulsion 
of  fat  globules  in  a  solution  of  caseinogen,  albumin,,  lactose 
and  salts,  and  rubber  latex,  in  which  the  continuous  phase 
is  also  a  solution  of  proteids,  while  the  disperse  phase 
consists  of  rubber  and  resin  globules  in  varying  pro- 
portions. As  regards  the  artificial  emulsions,  some  of 
them  are  unwelcome  by-products  of  various  industrial 
processes,  such  as  the  condense  water  from  steam  engines 
which  contains  a  portion  of  the  oil  used  for  cylinder 
lubrication  in  a  state  of  very  fine  and  persistent  division, 
or  wool  washings,  in  which  some  of  the  wool  fat  is 
emulsified  by  the  action  of  the  soaps  formed  from  a 
portion  of  the  fat.  A  large  number  of  emulsions  are 
also  prepared  purposely,  such  as  a  number  of  well-known 
pharmaceutical  preparations,  lubricating  compounds,  etc. 

The  simplest  systems  of  this  class  are  the  pure 
"  oil-water  "  emulsions,  which,  as  the  name  says,  contain 
no  soaps,  proteids  or  other  emulsifying  agents.  The  type 


VARIOUS  EMULSIONS.  .'{<) 

of  these  is  condense  water,,  with  about  one  part  of  oil  in 
ten  thousand  or  more  of  water ;  very  similar  emulsions 
may  be  obtained  by  pouring  a  dilute  alcohol  or  acetone 
solution  of  an  oil  into  a  large  volume  of  water,,  when  the 
oil  separates  in  the  form  of  very  minute  globules.  In 
this  way  it  is  possible  to  obtain  stable  emulsions  contain- 
ing up  to  one-thousandth  part  of  oil.  This  great  dilution 
is  quite  in  keeping  with  what  we  know  already  about  the 
suspensoids,  and  investigation  of  such  oil-water  emulsion^ 
which  has  been  carried  out  during  the  last  few  years  by 
Lewis,  Ellis  and  Goodwin,  and  the  author,  shows  that 
they  do  not  differ  materially  from  systems  with  solid 
particles.  The  oil  globules  show  active  Brownian  move- 
ment, are  coagulated  by  electrolytes  and  can  even — as 
was  first  showrn  by  the  author — be  retained  by  certain 
filtering  media,  for  instance  by  the  ultra-filters  mentioned 
in  a  previous  chapter.  This  possibility  proves  that  very 
small  liquid  particles  approaching  ultra-microscopic 
dimensions  possess  a  high  degree  of  rigidity,  a  conclusion 
fully  borne  out  by  mathematical  investigations  carried  out 
by  the  author. 

While,  therefore,  systems  of  two  liquid  phases 
containing  only  a  small  percentage  of  widely  separated 
particles  differ  in  no  material  respect  from  similar  systems 
with  rigid  particles,  a  very  important  difference  appears  as 
the  percentage  of  disperse  phase  increases.  With  rigid 
particles  this  percentage  is  obviously  limited  by  simple 
geometrical  considerations.  If  we  imagine  the  particles 
to  be  spheres  of  equal  diameter  and  to  be  so  numerous 
that  they  are  in  closest  contact — in  which  case  each 
sphere  touches  twelve  others — they  occupy  about  74  per 
cent,  of  the  total  volume.  Such  an  arrangement  of  solid 
particles  will,  however,  no  longer  have  the  properties  of  a 


40 


LIMITS     OF    PHASE    RATIO. 


liquid,  but  will  be  a  mud  or  paste  ;  filter  press  cakes  of, 
e.g.,  calcium  carbonate,  which  still  contain  about  40  per 
cent,  of  water,  or  only  60  per  cent,  instead  of  74  per  cent. 


FIG.  8. 

of  solid  matter,  certainly  no  longer  resemble  a  liquid. 
If  the  disperse  phase  is  liquid,  that  is,  still  easily 
deformed,  it  is  obvious  that  the  whole  system  will  still 


EMULSIFICATION  ;    EFFECT  OF  INTERFACIAL  TENSION.  4l 

retain  the  character  of  a  liquid,  even  if  the  disperse  phase 
occupies,  or,  indeed,  exceeds  74  per  cent,  of  the  total 
volume.  The  globules  will  touch  one  another  when  the 
former  figure  is  reached,  and  as  it  is  exceeded  will 
become  flattened  at  the  points  of  contact,  developing  the 
twelves  faces  of  a  dodecahedron.  There  is  thus  no  limit 
to  the  ratio :  volume  of  disperse  phase  /  total  volume, 
which  may  approach  unity.  As  a  matter  of  fact, 
emulsions  containing  99  per  cent,  of  oil  in  1  per  cent,  of 
soap  solution  have  been  made  by  Pickering. 

It  is  quite  impossible  to  prepare  emulsions  con- 
taining such  percentages  of  disperse  phase  (or  anything 
more  than  fractions  of  one  per  cent.)  unless  the  continuous 
phase  is  a  solution  of  certain  substances,  such  as  soap, 
various  products  of  the  saponification  of  albumin,  or  one 
of  the  saponins.  All  these  solutions  have  one  character- 
istic in  common :  they  froth  strongly  even  in  great 
dilutions.  Frothing,  which  never  occurs  with  pure 
liquids,  is  a  definite  indication  that  the  dissolved 
substance  lowers  the  surface  tension  of  the  solvent,  and 
the  process  of  emulsification  is  closely  connected  with  this 
lowering  of  surface  tension,  or  more  correctly  the  inter- 
facial  tension  between  the  two  phases.  This  connection 
can  be  very  easily  demonstrated  by  an  apparatus  devised 
by  Donnan,  and  illustrated  in  Fig.  8.  A  pipette  A  is 
provided  with  a  length  of  capillary  tube  B,  which  leads 
into  a  bend  C.  The  latter  is  drawn  out  into  a  point  D, 
which  is  ground  off  flat.  The  pipette  is  filled  with  the 
oil  to  be  examined,  and  the  outlet  submerged  in  the  liquid 
which  is  to  be  the  disperse  phase.  The  size  of  the  drop 
which  issues  from  the  point  D  is  settled  on  one  hand  by 
the  difference  in  specific  gravity  of  the  two  liquids,  and 
on  the  other  by  the  surface  tension  acting  round  the 


42  STRUCTURE     OF    EMULSIONS. 

circumference  of  the  point,  which  tends  to  retain  the 
drop.  Any  decrease  in  surface  tension  accordingly  shows 
itself  in  diminished  size,  or  increased  number  of  drops, 
and  this  increase,  and  the  obvious  parallelism  between  it 
and  the  emulsifying  power  of  the  liquid,  is  very  striking. 
While,  for  instance,  a  pipette  used  by  the  author  gave  65 
drops  of  light  petroleum  in  water,  it  gave  about  260  drops 
in  a  1  per  cent,  soap  solution.  With  such  a  soap  solution 
it  is  quite  easv  to  make  emulsions  containing  over  90  per 
cent  of  petroleum  as  disperse  phase,  and  this  percentage 
may  be  increased  still  further  by  certain  methods 
indicated  by  Pickering. 

We  can  now  form  a  picture  of  the  various  factors 
which  make  'possible  the  existence  of  these  high  per- 
centage emulsions,  and  which  give  them  their  character- 
istic properties.  The  oil  globules  are  no  longer  spherical, 
but  polyhedral,  the  adjoining  faces  being  separated  by 
very  thin  films  of  the  disperse  phase.  Such  films  would 
tear,  if  they  had  the  high  surface  tension  of  water,  and 
can  only  persist  if  the  interfacial  tension,  which  tends  to 
tear  them,  is  very  greatly  lowered  by  certain  dissolved 
substances.  If  the  films  are  thick,  i.e.,  if  the  particles  are 
widely  separated,  a  persistent  emulsion  is  possible,  as  we 
have  seen  above,  even  in  water.  We  also  realise  that 
such  a  system  consisting  of  polyhedra  of  oil  separated  by 
thin  films  of  an  aqueous  medium — or,  put  in  the  other 
wajr,  a  honeycomb-like  structure  of  the  latter  filled  with 
oil — must  have  the  high  viscosity  which  is  so  charac- 
teristic of  the  emulsions  with  a  large  percentage  of 
disperse  phase.  If  such  liquids  are  sheared,  the  polyhedra 
will  constantly  have  to  slide  over  one  another,  and  in  the 
process  will  be  streched  and  deformed.  In  other  words, 
the  total  surface  of  the  system  will  be  enlarged  and, 


STABILITY    AND    PREPARATION   OF   EMULSIONS.  43 

notwithstanding!  the  low  surface  tension,  a  considerable 
amount  of  surface  energy  developed,  which  appears 
disguised  as  viscosity.  Extreme  cases  are  presented  by 
some  of  Pickering's  emulsions  with  99  per  cent,  of  oil,  the 
viscosity  of  which  is  so  great  that  they  can  be  cut  into 
cubes  and  retain  their  shape. 

The  stability  of  emulsions  made  with  one  of  the 
emulsifying  agents  mentioned  above  varies  considerabljr. 
Generally  speaking,  they  are  destroyed  by  the  addition  of 
all  substances  which  affect  the  latter  :  thus  emulsions 
made  with  soap  solution  are  at  once  destroyed  by  the 
addition  of  small  amounts  of  acid,  which  decompose  the 
soap.  Rubber  latex  is  coagulated  by  such  substances  as 
coagulate  the  particular  proteids  which  it  contains,  for 
instance,  acetic  acid  in  the  case  of  Hevea  latex.  The 
phases  can  often  be  separated  without  the  use  of  chemical 
agents,  by  ceiitrifuging,  as  in  the  case  of  milk.  Finally, 
as  the  globules  are — generally  negatively — charged,  they 
travel  in  the  electric  field  and  this  method  has  at  least 
been  suggested  for  industrial  application. 

The  actual  methods  of  making  emulsions — suitable 
qualities  of  the  phases  being  assumed — deserve  only 
passing  mention.  The  two  components  are  shaken  up 
together  until  the  disperse  phase  is  sufficiently  finely 
distributed,  or  the  ''  oil  "  is  injected  into  the  other  liquid 
from  a  syringe  with  a  fine  nozzle.  Temperature  naturally 
plays  a  considerable  part  in  these  operations,  as  it  reduces 
the  interfacial  tension  between  the  phases,  and  also  their 
viscosities. 

We  now  proceed  to  the  consideration  of  the  second 
— and  by  far  the  more  important — class  of  colloids,  the 
emulsoids.  The  only  inorganic  emulsoid  of  much 
importance  is  silicic  acid,  which  was  very  exhaustively 


4-4>  EMULSOIDS.        SILICIC  ACID  SOL. 

studied  b}'  Graham.  If  a  solution  of  sodium  silicate 
("  waterglass  ")  is  poured  into  a  slight  excess  o.f 
hydrochloric  acid,  and  the  mixture  is  dialysed  until  the 
free  acid  and  the  sodium  chloride  have  been  removed' 
there  lemains  in  the  dialyser  a  perfectly  clear 
colourless  sol  of  silicic  acid.  This  sol,  either 
spontaneously  or  on  addition  of  electrolj'tes — 
carbonates  or  phosphates  being  particularly  effective 
— sets  to  a  bluish  and  almost  transparent  gel- 
No  water  separates,  i.e.,  the  gel  contains  the  same  amount 
of  water  as  the  sol,  and  the  transformation  is  irreversible, 
that  is,  the  gel  cannot  be  dissolved  again.  The  change  is 
also,  as  far  as  can  be  ascertained,  continuous  :  the 
viscosity  of  the  sol  increases  steadilj'  until  it  becomes  im- 
movable, this  feature  forming  an  important  distinction 
between  gel  formation  and  the  solidification  of  molten 
substances,  which  is  discontinuous.  Silicic  acid  sols  and 
gels  are  supposed  to  have  existed  in  considerable 
quantities  at  some  geological  periods,  and  various 
minerals,  like  agate  and  opal,  probably  owe  their  origin  to 
such  gels. 

The  organic  emulsoids  are  veiy  numerous  and 
extremely  important,  including,  as  they  do,  most  of  the 
proteins,  such  as  albumin,  casein,  gelatine  ;  a  number  of 
carbohydrates  as  starch,  agar,  the  gums,  cellulose  and  its 
various  nitro-  and  acetylderivatives  ;  the  soaps,  etc.  It 
is  impossible  within  the  limits  of  a  short  work  to  do  more 
than  to  select  a  few  typical  substances  and  to  develop  a 
few  general  points  of  view  to  which  their  extremely 
varied  behaviour  becomes  referable. 

Two  of  these  may  usefully  be  considered  together, 
viz.,  gelatine  and  agar.  The  former  is  a  protein,  while 
the  latter  is  a  mixture  of  carbohydrates,  the  most 


GELATINE  AND  AGAR  SOLS.  4-5 

characteristic  of  which  is  d-galactan.  Both  gelatine  and 
agar,  when  immersed  in  cold  water  imbibe  large  quantities 
and  swell,  until  an  equilibrium  is  attained.  On  heating, 
they  dissolve  to  sols,  the  temperature  necessary  to  effect 
this  being  25  — 35  for  gelatine,  according  to  the  con- 
centration, and  about  boiling  point  for  agar.  On  cooling 
these  sols  set  to  jellies,  the  setting  point  for  gelatine 
being  a  few  degrees  lower  than  the  '  melting  point," 
whereas  the  agar  sol  can  be  cooled  to  about  35  before 
setting.  The  process  is  completely  reversible,  but  the 
gels  have  to  be  heated  to  original  temperatures,  to  be 
transformed  into  sols  again.  Agar  is  thus  a  very  striking 
example  of  the  phenomenon  of  hysteresis.,  as  the  sol,  once 
formed,  does  not  gelatinise  until  cooled  about  35°, 
whereas  the  gel  is  not  re-transformed  into  sol  unless 
raised  to  a  temperature  nearly  70°  higher.  A  similar, 
but  very  much  slighter,  hysteresis,  is  noticeable  in  the 
case  of  gelatine.  As  in  the  case  of  silicic  acid  the  trans- 
formation is  quite  continuous,  that  is  (with  the  same 
reservation  regarding  the  difficulty  of  measurements  in 
the  neighbourhood  of  the  setting  point)  the  viscosity 
increases  continuously  as  temperature  falls. 

The  viscosity  of  gelatine  sols  has  been  the  subject  of 
careful  and  important  investigations,  particularly  by 
Garrett.  These  tend  to  show  that  a  gelatine  sol,  unlike  a 
homogenous  liquid,  has  not  a  definite  constant  viscosity  at 
a  given  temperature,  but  that  the  viscosity  varies  with  the 
velocity  of  shear,  and  alters  with  time  if  shearing,  even  at 
constant  velocity,  is  continued.  Similar  measurements  at 
very  low  rates  of  shears  have  been  carried  out  by  the 
author  and  his  students.  Without  entering  into  details  of 
these  investigations,  which  still  present  many  difficulties, 
it  may  be  said  that  the  behaviour  of  the  sol  is  inexplicable, 


46 


STRUCTURE    OF  EMULSOID  SOLS. 


except  on  the  assumption  that  it  is  a  system  ot  two  fluid 
phases,  or,  in  other  words,  that  it  consists  of  drops  or 
globules  having  a  high  gelatine  content,  in  a  continuous 
phase  which  is  a  dilute  solution.  The  process  of  sol 
formation  on  this  assumption  becomes  an  extension  of  the 
process  of  swelling  by  imbibition,  accompanied  by 
disintegration.  While  the  system  is  thus,  mechanically, 
one  of  two  liquid  phases,  it  must  be  emphasised  that  it 
differs  from  an  emulsion  of  two  insoluble  phases  by  the 
facility  with  which  the  solvent  may  be  shifted  from  one 
phase  into  the  other. 


CHAPTER  VI. 

A  behaviour  differing  entirely  from  that  of  gelatine  is 
shown  by  the  albumins,  which  are  perhaps  the  most 
important  among  the  emulsoids,  and  have  received  an 
amount  of  investigation  commensurate  with  their  import- 
ance. As  their  type  neutral  albumin  prepared  from  wrhite 
of  egg  may  be  taken.  This  is  soluble  in  water  at 
ordinary  temperature,  and  does  not  form  a  gel  either  with 
increasing  concentration  or  on  cooling.  On  the  other 
hand,  it  shows  a  phenomenon  we  have  not  so  far 
encountered :  it  coagulates  irreversibly  on  heating 
to  about  60°.  The  temperature  at  which  this  change 
occurs  may  be  altered  by  the  addition  of  salts,  and  can — 
as  has  been  shown  by  Pauli  and  Handowsky — be  raised 
so  much  by  the  addition  of  a  thiocyanate  that  the  sol 
does  not  coagulate  even  at  boiling  point. 

In  close  connection  with  this  property  of  the 
albumin  sol  is  its  behaviour  to  salts  in  the  cold.  On  the 
addition  of  salts  in  suitable  concentrations  the  albumin 
sol  becomes  turbid,  and  the  albumin  finally  settles 
out  in  flocculent  masses.  The  coagulum,  however,  shows 
different  characteristics  with  different  salts,  and  these,  if 
for  the  moment  we  consider  only  the  cation,  divide 
themselves  into  three  groups.  The  salts  of  the  alkalies 
and  of  magnesium  produce  coagulation  or  "salting  out" 
only  in  great  concentrations,  and  the  process  is  reversible 
— i.e..,  on  dilution  or  removal  of  the  salt  the  albumin 
again  goes  into  solution.  The  salts  of  the  alkaline  earths 
salt  out  in  similar  concentration,  but  the  precipitate 


48  ALBUMIN.      HOFME1STER    SERIES. 

becomes  insoluble  on  standing  even  for  a  short  time  ; 
while,  finally,,  the  salts  of  the  heavy  metals  salt  out 
irreversibly  even  in  low  concentrations. 

If  we  now  consider,  instead  of  a  variety  of  salts  with 
different  cations,  a  series  having  the  same  cation  but 
different  anions,  we  become  acquainted  with  a  sequence 
of  the  greatest  importance,  which  was  first  discovered  by 
Hofmeister  in  his  investigations  on  egg  albumin,  and  is 
generally  called  after  him.  Some  of  his  results  are  given 
in  the  following  table,  which  shows  the  concentration  in 
moles  per  litre  of  the  sodium  salts  of  various  acids 
necessary  to  salt  out  the  same  albumin  sol  at  30° — 40  . 

Citrate        0'56 

Tartrate         078 

Sulphate         0*80 

Acetate      1*69 

Chloride         3'62 

Nitrate        5'42 

Chlorate     5*52 

Iodide        j  Do  not  salt  out  in 

Thiocyanate       (   saturated  solution 

This  series  refers  to  the  neutral  albumin  investigated 
by  Hofmeister.  A  further  very  striking  fact  was  shown 
by  Pauli  :  the  effect  of  the  series  is  reversed  in  faintly 
acid  sols,  in  which  the  iodide  and  thiocyanate  have  the 
greatest,  and  the  citrates  and  tartrates  the  least  action. 

At  first  sight  no  explanation  whatever  of  these 
features  suggests  itself.  The  obvious  temptation  is  to 
look  for  a  chemical  action,  and  the  order  in  which  the 
acids  follow  one  another — tribasic,  dibasic  and  monobasic 
— gives  some  plausibility  to  that  view.  It  becomes, 
however,  quite  untenable  when  we  study  the  effect  of  the 


GENERAL    IMPORTANCE     OF    HOFMEISTER     SERIES.  If) 

series  on  other  emulsoids  besides  albumin,  for  instance, 
on  substances  as  dissimilar  chemically  as  gelatine  and 
agar.  As  far  as  a  comparison  is  possible  this  is  parallel 
with  the  effect  on  albumin.  The  addition  of  citrate  or 
tartrate  to  a  gelatine  or  an  agar  sol  raises  the  setting 
point  and  produces  a  stiffer  gel ;  the  addition  of  an  iodide 
or  a  thiocyanate  lowers  it  and  leads  to  the  formation  of 
gel  of  slighter  consistence.  With  sufficient  concentrations 
of  thiocyanate  a  gelatine  or  agar  sol  may  be  entirely 
prevented  from  setting  to  a  gel  at  ordinary  temperature. 
The  viscosity  of  the  sols  is  affected  in  the  same  sense. 
The  only  general  view  which  co-ordinates  all  these 
phenomena  is  this  :  they  are  all  various  manifestations  of 
a  change  in  the  distribution  of  \vater  between  the  two 
phases.,  and  the  salts  of  the  Hofmeister  series  affect  this 
distribution,  and  do  so  by  altering  the  compressibility  of 
water.  The  solution  of  most  emulsoids  proceeds  with 
contraction — a  point  which  we  shall  discuss  in  greater 
detail  when  dealing  with  gels — and  the  view,  propounded 
and  supported  with  a  great  deal  of  evidence  by  Freundlich, 
that  the  effect  of  the  Hofmeister  series  is  closely 
connected  with  changes  in  the  compressibility  of  water, 
is  therefore  entirely  rational. 

The  emulsoids  described  so  far,  namely  gelatine  and 
agar  on  one  hand  and  albumin  on  the  other,  are  of  two 
different  types  as  regards  their  behaviour  at  different 
temperatures.  The  first  named  set  to  gels,  without 
separation  of  water,  belowr  certain  temperatures,  wrhile  the 
latter  coagulates  irreversibly  above  a  certain  limit.  A 
very  large  number  of  emulsoids  do  not  exhibit  either  of 
these  characteristics :  they  neither  form  gels  at  low 
temperatures  nor  coagulate  on  heating.  This  class 
contains  practically  all  the  emulsoids  in  solvents 


50  VARIOUS    EMULSOID    SOLS. 

other  than  water,  and  many  substances  which  form  sols 
with  water,  such  as  the  gums,  starch,  etc.  Another 
member  of  this  group  is  casein,  which  is  peculiar  in  being 
insoluble  in  water  alone,  but  soluble  in  weak  solutions  of 
alkali.  The  alkali-casein  sols  are  in  many  ways  very 
typical  emulsoid  sols  :  they  show  a  very  rapid  increase  of 
viscosity  with  falling  temperature  and  increasing 
concentration,  without  either  gel  formation  or  heat 
coagulation.  The  effect  of  the  Hofmeister  series,  as  far 
as  it  has  been  examined,  is  the  same  as  on  other  sols,  and 
casein  also  shows  slight  contraction  in  solution. 

A  similar  behaviour,  that  is  an  absence  of  disconti- 
nuitjr  at  either  extreme  of  temperature,  is  shown  by  gum 
arabic  and  similar  sols.  Gum  arabic  can  be  "salted  out" 
by  great  concentrations  of  sodium  chloride  or  other  salts, 
the  whole  effect  being  again  obviously  due  to  removal  of 
water  from  one  phase  into  the  other. 

Among  the  sols  with  continuous  phases  other  than 
water  the  most  interesting  are  undoubtedly  the  sols  of 
cellulose  and  of  its  nitroderivatives,  which  have  assumed 
great  industrial  importance  in  recent  years,  as  the 
materials  for  the  production  of  artificial  silk,  etc.  As  is 
well  known,  cellulose  is  dissolved  by  cupric  oxide-ammonia 
solution,  forming  a  typical  emulsoid  sol  of  very  high 
viscosity,  from  which  the  cellulose  can  be  precipitated  as 
a  coherent  gel  by  neutralising  the  solvent  with  acid. 
Cellulose  is  also  gradually  dissolved  by  a  50  per  cent  zinc 
chloride  solution  at  about  60°,  and  this  sol  is  characterised 
by  an  even  higher  viscosity  than  the  cuprammonium  sol. 
The  nitro-celluloses — collodion  and  gun-cotton — form 
sols  with  a  whole  series  of  solvents,  such  as  glacial  acetic 
acid,  acetone,  ether-alcohol,  amyl  acetate,  etc.,  all  of  which 
show  the  typical  emulsoid  properties  :  very  high,  but 


ELECTRICAL   PROPERTIES  OF  EMULSOID  SOLS.  5l 

inconstant,  viscosities,  and  slight  turbidity  or  opalescence. 
With  one  of  these  sols,  the  acetic  acid  collodion,  we  are 
already  familiar  as  a  material  for  making  ultra  filters : 
these  sols  leave  a  coherent  gel  like  mass  when  the  acetic 
acid  is  removed  by  water.  Many  of  the  other  nitro- 
cellulose sols  leave  coherent  films  on  drying,  the  properties 
of  which  can  be  modified  by  various  additions,  such  as 
castor  oil  or  camphor. 

No  special  reference  has  so  far  been  made  to  the 
electrical  condition  of  the  emulsoids,  but  from  what  has 
been  stated  in  regard  to  salting  out  and  related  phenomena 
it  is  obvious  that  there  is  a  profound  difference  between 
the  suspensoids  and  emulsoids  in  this  respect.  The 
electric  properties  of  the  latter  are  not  nearly  as 
unambiguous  as  those  of  the  former,  and  are  affected,  or 
indeed,  determined  almost  entirely  by  the  reaction  of  the 
dispersion  medium — i.e.,  the  concentration  of  H  or  HO 
ions.  This  point  also  has  been  investigated  chiefly  in 
connection  with  albumin  sols,  especially  by  Pauli  and  his 
pupils.  He  showed  that  albumin  freed  as  far  as  possible 
from  electrolytes  by  prolonged f  dialysis  and  by  freezing 
and  thawing  did  not  travel  in  the  electric  field,  i.e.,  was 
not  charged.  In  the  presence  of  small  concentrations  of 
acid  it  assumes  a  positive  charge,  and  in  the  presence  of 
alkali  a  strong  negative  one.  The  conditions,  however, 
are  much  more  complicated  than  in  suspensoids,  as 
chemical  reactions  between  bodies  as  sensitive  as  albumin 
and  the  added  electrolyte  must  be  expected,  while  they 
are  not  at  all  probable  in  many  suspensoids. 

The  optical  behaviour  and  the  ultra-microscopic 
appearance  of  the  emulsoids  require  only  passing  reference. 
The  sols  are  all  either  slightly  turbid  or  at  least 
opalescent  and  show  a  marked  Tyndall  cone.  In  the 


52  SEMI-COLLOIDS. 

ultra-microscope  they  cannot  generally  be  resolved,  that 
is  they  show  no  particles,  but  only  diffused  light.  As, 
apart  from  possessing  a  minimum  size,  particles  to  be 
visible  must  differ  either  in  colour  or  in  refractive  index 
from  the  dispersion  medium,  their  absence  is  easily 
accounted  for,  the  difference  between  the  two  phases — a 
concentrated  disperse  and  a  dilute  continuous  phase — may 
conceivably  be  very  slight. 

While  there  is  thus  a  fairly  sharp  demarcation 
between  the  suspensoids  and  the  emulsoids,  there  is 
a  very  gradual  transition  from  the  latter  to  true  solutions. 
We  know  a  whole  range  of  substances  the  colloidal 
character  of  which  is  less  and  less  marked,  appears  only  in 
certain  solvents  or  at  certain  concentrations,  and  is  often 
confined  to  one  or  the  other  characteristic  of  emulsoid 
sols.  Among  these  semi-colloids  may  be  mentioned  the 
soaps  —  i.e.,  the  oleates,  stearates  and  palmitates  of  the 
alkalies.  In  dilute  aqueous  solution  they  show  a  slight 
lowering  of  the  vapour  tension,  which  decreases  with 
increasing  concentration  and  disappears  entirely  at  about 
one  mole  (about  28  per  "cent,  of  sodium  palmitate).  At 
that  concentration  we  have  a  typical  emulsoid  sol,  which 
sets  to  a  jelly  on  cooling  and  can  be  '  salted  out,"  as  is, 
indeed,  done  industrial!}'.  In  alcoholic  solution,  on  the 
other  hand,  the  soaps  show  normal  raising  of  the  boiling 
point  and  all  characteristics  of  true  solutions. 

The  colloidal  character  is  still  less  marked  in  some  of 
the  products  of  cleavage  of  albumin,  for  instance  in  the 
peptones.  They  diffuse  slowly  but  appreciably  and  show 
perceptible  lowering  of  the  freezing  point :  the  only 
properties  which  link  them  to  the  emulsoids  are  slight 
turbidity  and  the  syrup  consistence  which  they  assume 
with  increasing  concentration. 


DYE     STUFFS. 


The  dyestuffs,  finally,  show  every  possible  variety  of 
behaviour.  Many  of  them,  like  eosin  and  methylene 
blue,  appear  "optically  void"  in  the  ultra-microscope, 
diffuse  and  lower  the  freezing  point  normally.  Others, 
like  congo  red  or  benzo-purpurin,  do  not  diffuse  and  show 
particles.  Others  again  have  emulsoid  character,  like 
fuchsin,  which  in  concentrated  solutions  forms  membranes 
on  the  surface  and  can  be  "salted  out"  with  concen- 
trated sodium  chloride.  A  very  exhaustive  investigation 
of  fifty  dyestuffs  of  the  indicator  class  has  been  carried 
out  by  Wolfgang  Ostwald,  from  which  it  appears  that  in 
practically  all  cases  either  the  base  or  the  acid,  if  not  the 
dye  itself,  shows  particles  or  at  least  a  cone  in  the  ultra- 
microscope. 


CHAPTER  VII. 

REFERENCE  has  already  been  made  in  the  preceding 
chapter  to  the  changes  which  sols  like  those  of  gelatine 
and  agar  undergo  on  cooling,  or  which  takes  place  in 
silicic  acid  sol  on  standing  or  the  addition  of  electrolytes. 
The  sols  in  these  conditions  set  to  coherent  gels  without 
loss  of  water  and  notwithstanding  the  very  large  amount 
of  the  latter  present  in  some  of  them — 2  per  cent  agar 
makes  a  very  stiff  gel — they  possess  some  of  the  properties 
of  solids.  This  very  striking  fact,  which  at  first  sight  is 
inexplicable,  becomes  intelligible  to  some  extent  when  we 
remember  organic  structures,  like  the  stems  of  plants, 
which  show  considerable  rigidity  and  elasticity  although 
containing  80  to  90  per  cent,  of  water ;  these  qualities 
are  due  to  the  liquid  being  enclosed  in  cells  and  vessels. 
We  are  probably  not  straining  the  analogy  too  much 
when  we  assume,  even  without  further  evidence — of 
which  there  is  a  considerable  amount— that  the  gels  must 
have  some  structure  of  this  kind,  that  is  to  say,  the 
liquid  must  be  enclosed  in  cells  formed  by  a  solid  phase. 
We  shall  return  to  this  important  question  after  the 
discussion  of  a  few  typical  cases. 

The  gels  are  generally  divided  into  "  elastic  "  and 
"  rigid "  gels,  gelatine,  agar,  the  nitrocellulose-gels 
belonging  to  the  former  class,  while  silicic  acid  gel  is  the 
chief  representative  of  the  latter.  The  terms  are  not 
strictly  correct,  as  the  silicic  acid  gel  can  be  felt  to 


DK-HYDRATION  OF  INELASTIC  GELS 


55 


3.0 


2.5 


2.0 


1.5 


1.0 


0.5 


B 


/ 


4  8  12 

FIG.  9. 

vibrate  when  the  vessel  containing  it  is  struck,  and  must 
therefore  possess  some  degree  of  elasticity,  but  they  are 
sufficiently  descriptive  to  be  generally  accepted.  The 


56  DE-HYDRATION    OF    INELASTIC    GELS  (continued). 

silicic  acid  gel  can  be  obtained  in  its  pure  form  by 
allowing  a  dialysed  sol  to  set  (a  process  which  can  be 
very  much  accelerated  by  bubbling  carbon  dioxide  through 
it  for  a  short  time)  and  then  forms  an  almost  clear, 
faintly  bluish  jelly,,  containing  up  to  30  moles  of  water  for 
one  of  SiOs,  i.e.,  about  90  per  cent  of  the  total  weight  of 
the  gel. 

If  a  slight  excess  of  waterglass  solution  of  about  1'16 
sp.  gr.  is  added  to  hydrochloric  acid  containing  about 
8  per  cent  HC1,  the  mixture  sets  almost  immediately  to  a 
white  opaque  gel.,  which  of  course  contains  the  excess  of 
sodium  silicate  and  the  sodium  chloride  formed  by  the 
reaction,  but  may  nevertheless  be  used  to  demonstrate 
many  of  the  properties  of  the  gel. 

The  pure  gel,  if  left  in  air,  rapidly  loses  water  even 
at  ordinary  temperature,  but  still  retains  several  moles. 
These  can  be  removed  by  drying  over  sulphuric  acid,  and 
the  course  of  dehydration  has  been  studied  by  van 
Bemmelen  in  a  series  of  most  exhaustive  and  careful 
experiments,  prolonged  in  one  instance  for  two  years. 
The  gels  were  kept  in  desiccatros  over  sulphuric  acid  of 
known  concentration  and,  therefore,  vapour  tension,  and 
weighed  at  frequent  intervals.  The  re-hydration  was 
also  investigated,  and  the  complete  results  covering  the 
range  from  3  to  0  moles  of  water  are  plotted  in  Fig  9  in 
which  the  abscissae  are  the  vapour  tensions  in  mm.  of 
mercury,  and  the  ordinates  the  amount  of  water  in 
moles. 

The  most  important  point  brought  out  is  that  there 
are  no  definite  hydrates,  but  there  is  a  continuous  loss  of 
water  and  an  equilibrium  corresponding  to  every  given 
vapour  tension.  In  this  respect  the  gel  differs  radically 
from,  say,  crystals  with  water  of  crystallisation.  Copper 


ADSORPTION    COMPOUNDS.  57 

sulphate,  e.g.,  crystallises  with  5  molecules  of  water, 
four  of  these  are  given  off  at  100°  and  the  fifth  at 
200  .  A  dehydration  curve  plotted  with  the  tempera- 
tures as  abscissae  and  the  water  contents  as  ordinate& 
would  therefore  consist  of  two  vertical  lines — the  process 
is  discontinuous. 

We  meet  here  for  the  first  time  a  class  of  com- 
pounds which  are  quite  definite  under  given  conditions, 
but  in  which  the  ratio  of  the  constituents  can  change 
continuously,  and  not  only  by  steps  corresponding  to- 
simple  stoichiometric  ratios.  Such  compounds,  known  as 
adsorption  compounds,"  are  of  enormous  importance  in 
nature,  and  will  be  further  considered  in  connection  with 
the  laws  of  adsorption. 

The  gel  gradually  undergoes  a  striking  change  in 
appearance  during  drying,  which  deserves  mention,  as  it 
throws  some  light  on  the  question  of  gel  structure.  At 
A  the  gel  is  still  bluish  and  translucent,  while  at  B  it 
becomes  chalky-white  and  opaque.  On  still  further 
drying,  however,  it  becomes  clear  again,  and  is  almost 
transparent  in  the  last  part  of  the  dehydration  curve. 
We  shall  again  refer  to  the  probable  meaning  of  these 
changes. 

The  elastic  gels,  like  gelatine,  have  not  been  investi- 
gated in  the  same  exhaustive  manner,  but  ordinary 
experience  already  shows  that  their  behaviour  when 
taking  up  or  losing  water  is  entirely  different  from  that  of 
the  rigid  gel.  Anyone  familiar  with  photograph}',  for 
instance,  knows  that  the  gelatine  film  does  not  become 
opaque  at  any  stage  of  imbibition  or  diying  process. 
Another  difference  is  noticeable  in  the  behaviour  of 
gelatine  in  water  and  in  water  vapour ;  the  amount  of 
water  taken  up  is  not  the  same  in  both  cases.  Experi- 


,')8  SWELLING  OF  ELASTIC  GELS 

ments  on  this  point  were  made  by  Schroeder,  who  kept  a 
gelatine  plate,  weighing  dry  0'904  gramme,  in  air 
saturated  with  moisture  for  eight  days,  at  the  end  of 
which  it  had  taken  up  0'37  gramme  of  water,  the  weight 
then  remaining  constant.  The  plate  was  then  submerged 
in  water  at  the  same  temperature  and  took  up  a  further 
5  63  grammes  in  one  hour.  If  the  gel  is  now  placed  into 
a  dry  atmosphere,  it  rapidly  loses  water  at  first — the 
speed  of  evaporation  being  at  first  almost  as  great  as 
from  a  free  water  surface — then  very  slowly,  until  an 
equilibrium  depending  on  the  vapour  tension  is  finally 
reached. 

This  absorption  of  water  by  elastic  gels,  and  the 
volume  changes  which  accompany  it,  are  of  enormous 
importance  physiologically,  and  have  accordingly  been 
much  investigated,  in  the  first  instance  by  botanists. 
The  following  are  the  salient  features  of  the  phenomenon  : 
when  an  elastic  gel  is  placed  in  water,  the  gel  imbibes 
some  of  it  and  swells,  but  the  total  volume  gel  plus  water 
decreases,  i.e.,  the  process  is  accompanied  by  compression. 
This  can  be  shown  in  a  variety  of  ways,  of  which  the 
following,  used  by  the  author,  is  perhaps  the  most 
striking.  One  gramme  of  the  gel  to  be  examined  is 
placed  in  an  ordinary  pycnometer,  the  latter  filled  with 
water  and  the  whole  weighed.  The  pycnometer  is  then 
placed  under  water,  and  left  till  the  gel  has  swelled  to  the 
full  extent,  taken  out,  dried  and  weighed  again.  The 
excess  represents  the  amount  of  water  which  has  entered 
the  pycnometer  owing  to  the  reduction  of  the  combined 
volume  gel  plus  water.  In  one  experiment,  in  which 
1  gr.  of  gum  tragacanth  was  used,  in  a  50  c.c.  pycnometer, 
the  increase  in  weight  after  one  week  amounted  to  0'9 
grammes  of  water,  i.e.,  0'9  c.c.  of  water  had  entered,  or 


LIBERATION  OF  HEAT  DURING  SWELLING.  59 

almost  2  per  cent,  of  the  original  volume.  To  obtain  the 
same  effect  by  compressing  the  water,  a  pressure  of  about 
400  atmospheres  would  be  necessary,  and  it  follows  at 
once  that  the  process  of  swelling  must  be  accompanied  by 
the  liberation  of  heat.  This  is,  indeed,  the  case,  and 
experiments  have  been  made  by  many  investigators  to 
ascertain  the  amount  of  heat  liberated  during  the  swelling 
of  gels.  The  following  table  gives  the  results  obtained 
by  Wiedemann  and  Luedeking,  in  gramme-calories  per 
gramme  of  (dry)  gel  : — 


Gel. 

Gramme-Calorie 
per  gr.  of  gel. 

Starch     

6'6 

Criiiri  nrubic       ...         ..< 

9'0 

Gum  trasacunth  .. 

10'3 

While  the  total  volume  gel  plus  water  decreases, 
that  of  gel  plus  imbibed  water  increases,  i.e.,  the  gel 
swells.  The  increase  in  volume  can  of  course  be 
measured  only  by  confining  the  gel  in  such  a  way  that  the 
water  has  access  to  it,  but  is  not  implicated  in  the  volume 
changes ;  for  instance,  by  placing  circular  discs  into  a 
cylinder,  which  they  fit  exactly,  and  placing  on  top  a 
weighted  piston  with  numerous  small  perforations, 
through  which  water  reaches  the  gel.  Experiments  with 
an  apparatus  of  this  description  have  been  made  by 
Reinke  with  the  foliage  of  Laminaria,  a  seaweed,  which 
behaves  almost  exactly  like  a  gel.  The  table  gives  the 
pressure  on  the  piston  in  atmospheres  (kg.  per  sq.  cm.) 
and  the  percentage  increase  in  volume  which  the 
imbibition  of  water  produces. 


60  NUMERICAL  VALUES  OF  SWELLING. 

Pressuiein  Atm.  Percentage  Increase 

of  Volume. 

41'2  16 

31'2  23 

21-2  35 

11-2  89 

7-2  97 

3'2  205 

1'2  318 

TO  330 

This  table  also  illustrates  what  large  amounts  of 
energy  enter  into  the  process.  Even  against  the 
enormous  pressure  of  over  42  atmospheres  the  gel  still 
expands  16  per  cent.,  while  with  a  pressure  of  1  atmo- 
sphere the  expansion  amounts  to  330  per  cent.  In  other 
words,  1  cubic  centimetre  of  gel,  if  the  swelling  takes 
place  in  one  dimension  only  (as  in  Reinke's  apparatus) 
will  lift  1  kilogramme  3 '3  cm.  Conversely  it  becomes 
clear  what  enormous  pressures  are  necessary  to  remove 
the  last  traces  of  water  from  a  gel,  as  the  present  example 
still  retains  16  per  cent,  under  a  pressure  of  42  atmo- 
spheres. 

The  various  physical  constants  of  gels — coefficient  of 
thermal  expansion,  modulus  of  elasticity,  the  optical 
constants — are  of  great  interest,  as  they  differentiate  the 
gels  fairly  sharply  from  both  liquids  and  solids,  but  can 
only  receive  passing  consideration  here,  as  throwing  light 
on  their  structure.  If  gels  are  not  strained,  they  are 
isotropic,  i.e.,  they  have  the  same  coefficient  of  expansion, 
modulus  of  elasticity  and  refractive  index  in  all  directions. 
The  coefficient  of  expansion  in  these  circumstances  is 
practically,  and  over  a  fair  range,  that  of  the  liquid 
contained  in  the  gel.  If,  however,  a  gelatine  gel  is 
stretched,  it  shows  very  remarkable  anomalies  ;  on  rapid 
warming  it  contracts,  while  rapid  cooling  produces 


ELASTIC    PROPERTIES   AND  STRUCTURE.  61 

expansion.  It  is  worth  noting  that  india-rubber, 
stretched  beyond  a  certain  limit,  shows  exactly  the  same 
behaviour,,  namely  a  negative  coefficient  of  expansion,  a 
fact  which  had  already  been  observed  by  Tyndall.  This 
behaviour  can  best  be  explained  by  regarding  the  system 
as  consisting  of  two  phases  with  a  cellular  structure,  but  a 
rigid  mathematical  treatment  presents  almost  insuperable 
difficulties. 

As  regards  the  elastic  properties,  the  most  important 
point  is  that  gels  are  deformed  without  change  of  volume; 
if,  for  instance,  a  gelatine  cylinder  is  stretched,  the  cross 
section  decreases  in  the  same  ratio  in  which  the  length 
increases,  so  that  the  product,  i.e.,  the  volume,  remains 
constant.  Here,  as  in  thermal  expansion,  the  properties 
of  the  liquid  portion  of  the  gel  are  again  seen  to  prevail. 

Under  stress,  gels,  which  normally  are  isotropic, 
become  doubly  refracting,  a  property  which  they  share 
with  solids,  such  as  glass.  The  change  from  simple  to 
double  refraction  has  been  utilised  to  study  their  elastic 
properties,  a  subject  into  which  it  is  not  possible  to  enter 
here. 

We  have  already  touched  on  the  question  of  gel 
structure  at  the  beginning  of  the  preceding  chapter, 
where  the  view— based  on  a  somewhat  crude  analogy — 
was  expressed  that  gels  were  systems  having  a  solid 
continuous  phase,  which  enclosed  or  contained  the  liquid 
phase.  It  must  be  pointed  out  that  the  term  solid  "  is 
here — and,  indeed,  throughout  our  consideration  of  the 
whole  subject — employed  in  a  somewhat  wider  sense  than 
is  usual.  It  is  intended  to  convey  only  that  the  phase  to 
which  it  is  applied  is  much  less  readily  deformable  than 
the  liquid  phase,  without  postulating  all  the  properties, 
such  as  elasticity  of  shear,  characteristic  of  a  solid.  The 


O2  GEL  STRUCTURE. 

formation  of  reversible  gels,  like  those  of  agar  or  gelatine, 
must  therefore  be  accompanied  by  a  change  in  the 
distribution  of  water,  and  we  may  naturally  expect  it  to 
be  affected  by  the  presence  of  dissolved  substances  which 
modify  this  distribution,  as  explained  in  the  chapter  on 
emulsoids.  This  anticipation  is  found  to  be  correct  ;  salts 
like  the  citrates  or  sulphates  produce  more  rigid  gels,  while 
iodides  or  thiocyanates  retard  or  prevent  gel  formation. 

As  regards  direct  evidence  of  structure  in  gels,  a 
considerable  amount  of  material  has  been  collected  by 
Biitschli,  who  examined  and  photographed  numerous 
examples  at  very  high  magnifications,  in  some  instances 
over  4,000  diameters.  The  photographs  very  generally 
show  a  structure,  which  is  interpreted  as  resembling  a 
honeycomb,  i.e.,  it  appears  to  consist  of  polyhedral  cells. 
Such  cells  were,  for  instance,  visible  in  Van  Bemmelen's 
silicic  acid  gels  while  they  were  turbid,  but  disappeared  at 
the  stage  at  which  the  gel  became  transparent  (see  the 
preceding  chapter).  The  explanation  given  by  Biitschli  is 
that  the  cell  walls  are  so  thin,  compared  with  the  wave 
length  of  light,  as  to  be  invisible  until  a  film  of  water  is 
formed  on  them  by  further  hydration. 

The  interpretation  of  Biitschli' s  results  has,  however, 
always  offered  great  difficulties  into  which  it  is  riot 
possible  to  enter  here,  but  which  are  inherent  in  the 
formation  of  microscopic  images  of  periodic  structures 
whose  order  of  magnitude  is  that  of  the  wrave  length  of 
light.  More  recent  investigations  with  the  aid  of  the 
ultra-microscope,  carried  out  by  Zsigmondy  and  by 
Bachmann,  appear  to  show  that  the  honeycomb  formation 
shown  by  Biitschli 's  photographs  is  not  the  ultimate 
structure  of  the  gel,  and  that  this  latter  is  ultra-  or  even 
amicroscopic. 


DIFFUSION    IX   GELS.  6$ 

Nevertheless,  the  existence  of  a  gel  skeleton — that 
is.  a  continuous  solid  phase — can  hardly  be  doubted,  and 
is  supported  not  only  by  the  reasonings  already  referred 
to,  but  also  largely  by  the  phenomena  of  diffusion  in  gels. 
That  dissolved  substances  diffused  into  or  out  of  gels  was 
known  already  to  Thomas  Graham,  and  is  a  fact  familiar 
to  every  photographer,  as  the  various  processes  of 
developing,  fixing,  toning,  etc.,  are  made  possible  only  by 
the  diffusion  of  the  respective  solutions  into  the  gelatine 
gel  containing  the  silver  haloid.  Graham  found  that 
sodium  chloride  diffused  in  gelatine  with  almost  the  same 
velocity  as  in  water,  but  this  has  been  shown  to  be  the  case 
only  with  fairly  dilute  gels.  In  concentrated  gels  the 
rate  of  diffusion  is  considerably  slower  than  in  liquids,  and 
it  can  be  varied  by  the  addition  of  various  substances  to 
the  gel.  This  has  been  shown  particularly  by  Bechhold 
and  Ziegler,  who  found  that  the  addition  of  sodium, 
sulphate,  glucose,  alcohol  or  glycerine  retarded  diffusion, 
while  urea,  iodides  and  chlorides  accelerated  it.  These 
substances  affect  the  distribution  of  water  between  the 
two  phases  and  therefore  probably  the  relative  volumes  of 
the  gel  walls  and  the  free  liquid.  It  is  probable  that 
diffusion  takes  place  almost  exclusively  in  the  latter,  as 
appears  from  another  experiment  by  Bechhold,  in  which  a 
gelatine  gel  containing  sodium  chloride  was  covered  with 
an  aqueous  solution  of  silver  nitrate  of  equivalent 
concentration.  In  these  cirzumstances  an  extremely  thin 
layer  of  silver  chloride  is  formed,  which  yet  prevents  .any 
further  diffusion.  It  can  be  shown  that  the  gel  skeleton 
remains  unaffected,  and  that  the  silver  chloride,  which  is 
sufficient  to  prevent  further  diffusion,  is  formed  only  in  the 
interstices,  or  in  other  words,  that  diffusion  only  takes 
place  in  the  latter,  i.e..  the  liquid  phase. 


64>  REACTIONS  IN  GELS.         LIESEGANG's  RINGS. 

This  experiment  leads  us  to  the  consideration  of  a 
further  important  section  of  our  subject,  that  of  reactions 
in  gels.  If  a  gel  contains  a  substance  in  solution,  and  a 
second  solution  capable  of  reacting  with  the  former  is 
allowed  to  diffuse  into  it,  reaction  takes  place,  but  exhibits 
a  number  of  highly  interesting  peculiarities.  The  most 
striking  one  is  that,  in  very  many  cases,  the  reaction  does 


FIG.  10. 

not  proceed  continuously,  but  that  the  product  is  deposited 
in  strata  separated  by  apparently  clear  intervals.  The 
phenomenon  was  discovered  by  R.  E.  Liesegaiig  (after 
whom  it  is  generally  called)  in  the  following  manner  :  A 
drop  of  silver  nitrate  solution  was  placed  on  a  film  of 
gelatine  gel  containing  potassium  bichromate.  The  silver 
bichromate  which  is  formed  by  the  reaction  is  not 
deposited  in  a  continuous  zone  round  the  drop,  but  in 


HKACTIONS   IN   (iKLS.  6.5 

concentric  rings  separated  by  apparently  clear  intervals. 
A  similar  structure  is  illustrated  in  Fig.  10  ;  the  tfst 
tubes  were  partly  filled  with  1  %  agar  gel  containing 
calcium  chloride,  and  on  this  were  poured  solutions  of 
sodium  carbonate,  the  ratio  of  the  concentrations  being 
4  :  2  :  \.  The  calcium  carbonate  formed  by  the  interac- 
tion is  deposited  in  a  number  of  strata,  and  the  effect  of 
the  different  concentration  of  sodium  carbonate  is  strikingly 
shown. 

No  satisfactory  explanation  of  these  stratifications 
can  be  given  at  present,  though  a  great  number  of  factors 
which  may  contribute  to  their  formation  have  been 
pointed  out  by  various  observers.  This  point  is  one 
which  may  be  expected  to  receive  increased  attention  in 
the  future,  in  view  of  the  great  frequency  of  such 
periodic  structures  in  nature,  and  the  difficulty  of 
explaining  them  on  any  other  basis. 

Another  feature  of  great  interest  has  been 
investigated  chiefly  by  the  present  writer.  By  choosing 
suitable  concentrations  of  the  gel  itself,  and  of  the 
reacting  salts,  most  of  the  insoluble  precipitates  in  the 
gel  can  be  obtained  in  very  large  crystals  ;  in  many  cases, 
as  with  lead  chloride,  lead  sulphate,  lead  chromate,  lead 
iodide,  calcium  sulphate,  barium  silico-fluoride  and  others, 
they  are  of  macroscopic  size.  Many  compounds  also  show 
a  marked  tendency  towards  the  formation  of  spherical 
aggregates  and  perfect  spherolites,  giving  the  well-known 
appearance  between  crossed  Nicols,  are  frequent.  Figs. 
1 1  and  1 2,  for  instance,  show  calcium  sulphate  obtained 
in  two  different  gelatine  gels,  magnified  100  diameters. 
Calcium  sulphate  obtained  by  reaction  in  aqueous  solutions 
of  similar  concentration  forms  fine  needles,  which,  on  the 
same  scale  would  appear  3  to  5  mm.  long. 

3 


OO          RE  ACTONS   IN  GEL.       THE     PRODUCTS  OF     REACTION. 

These  results  show  that  large  aggregates  of  very 
insoluble  substances  can  be  formed  by  reactions  in  a 
gelatinous  medium  in  comparatively  short  times  ;  they  thus 
throw  considerable  light  on  many  questions  of  physiology, 
pathology  and  geology,  detailed  discussion  of  which, 
however,  is  beyond  the  scope  of  these  articles. 


FIG.  11. 

The  gels  described  so  far  are  all  formed  by  substances 
which  are  eminently  colloids  in  Graham's  sense,  i.e.,  bodies 
which  have  never  been  prepared  in  a  crystalline  form. 
Some  cases  are,  however,  known  in  which  transient  gels 
are  formed  by  substances  which  crystallize  quite  well. 
Several  instances  are  *  known  to  organic  chemists  of 
reactions,  in  which  a  new  phase  first  appears  as  a  jelly, 


UNSTABLE   GKLS. 


67 


which  eventually  breaks  up  and  deposits  crystals.  The 
author  has  examined  two  compounds  of  widely  different 
constitution  which  show  gel  formation  to  a  remarkable 
degree.  The}'  consist  of  perfect  crystals  and  are  sparingly 
soluble  in  cold,  more  readily  in  hot,  organic  solvents.  On 
rapid  cooling  of  the  hot  solutions  no  separation  of  crystals 


FIG.  12. 

takes  place,  but  they  set  to  very  perfect  gels,  which, 
however,  are  not  stable.  After  a  shorter  or  longer  time 
they  begin  to  liquefy  and  crystals  of  the  substance  appear. 
There  transformations  are  of  great  theoretical  interest,  as 
possibly  throwing  light  on  gel  formation  in  general,  and 
explaining  the  formation  of  crystalline  minerals  from  gels, 
but  cannot  be  discussed  in  detail  here. 


CHAPTER  VIII. 

We  have  already  had  occasion  to  emphasise  the 
importance  of  the  large  boundary  surface  between  the 
phases  of  a  disperse  system,  inasmuch  as  it  influences  its 
electrical  properties  and,  in  the  case  ot  two  liquid  phases, 
its  viscosity.  We  now  have  to  consider  a  very  large  and 
very  varied  class  of  phenomena,  in  which  this  surface  is 
the  determining  factor,  all  of  which  may  be  comprehen- 
sively described  as  changes  of  concentration  in  one  phase 
at  its  boundary  surface  with  another  phase. 

Many  instances  will  be  familiar  to  the  reader,  either 
from  text-books  or  from  actual  use.  Such  are,  for 
instance,  the  capacity  of  charcoal  to  condense  large 
volumes  of  gases,  of  which  advantage  is  taken  for  obtain- 
ing extremely  high  vacua.  A  similar  property  of  charcoal, 
that  of  taking  out  of  solutions  colouring  matter  or  the 
higher  alcohols  constituting  "  fusel  oil,"  has  been  known 
since  the  end  of  the  eighteenth  century  and  is  used 
industrially  on  a  very  large  scale  ;  china  clay  and  fuller's 
earth  are  also  employed  for  purposes  of  the  same  kind. 
Another  phenomenon  of  the  same  description  is  the  power 
possessed  by  certain  gels,  such  as  gelatine  and  isinglass,  of 
taking  down  turbidities  in  organic  solutions.  An  instance 
familiar  to  the  analyst  is  the  well-known  fact  that  the 
concentration  of  many  solutions,  e.g.,  of  lead  salts,  is 
considerably  reduced  by  filtration  through  paper,  that  is, 
by  contact  with  cellulose  fibre. 

This  list  could  be  extended  very  largely,  but  the 
instances  given  are  sufficient  to  show  that  in  all  cases  the 


SURFACE  TENSION   AND   ENERGY.  6.9 

effect  is  produced  at  a  boundary  surface  of  considerable 
extent  ;  charcoal,,  clay,  and  even  more  strikingly  the  gels 
are  all  characterised  by  a  very  large  surface  development, 
Certain  substances  in  solution  are  concentrated  at  these 
surfaces,  and  this  change  in  concentration  is  now 
generally  called  adsorption. 

The  occurrence  of  these  changes  at  surfaces  also 
provides  us  with  the  clue  to  their  investigation.  We  have 
already,  when  discussing  emulsions,  referred  to  the  fact 
that  the  surface  of  a  liquid  against  its  own  vapour,  or 
against  a  second  liquid,  is  in  tension,  known  as  surface 
tension  in  the  former  and  as  interfacial  tension  in  the 
latter  case.  This  tension  is  an  extremely  well-defined 
physical  constant,  and  can  be  measured  by  a  great 
number  of  methods.  For  details  of  these  the  reader  must 
be  referred  to  the  text-books  of  physics  or  physical 
chemistry ;  they  all  depend  on  the  tendency  of  the 
surface  tension  to  reduce  the  surface  and  to  establish 
equilibrium  with  the  other  forces  acting  on  the  body  of 
liquid  under  examination.  As  it  takes  work  to  produce 
or  to  enlarge  a  surface,  it  is,  when  formed,  the  seat  of 
energy,  which  is  measured  by  the  product  of  surface  and 
surface  tension  per  unit  length,  and  which,  of  course, 
tends  to  become  a  minimum.  We  already  know  of  one 
way  in  which  this  may  be  accomplished,  viz.,  by  the 
surface  becoming  a  minimum  ;  thus  a  drop  of  liquid 
suspended  in  another  of  the  same  specific  gravity  assumes 
spherical  shape,  the  sphere  having  the  minimum  surface 
for  a  given  volume.  Obviously  this  is  possible  only  where 
both  phases  are  easily  deformable,  i.e.,  when  both  are 
liquid,  or  one  liquid  and  the  other  gas. 

The  question  now  arises,  whether  such  tensions  and 
energies  also  exist  at  the  surfaces  gas-solid  and  liquid- 


70      REDUCTION   OF  SURFACE   ENERGY    AS    CAUSE  OF   ADSORPTION. 

solid,  especially  in  view  of  the  instances  quoted  above,  all 
of  which  refer  to  such  systems.  It  is  obvious  that  the 
immediate  methods  of  demonstrating  and  measuring  such 
tensions,  as  applied  to  easily  deformed  phases,  fail,  since 
the  surface  of  the  solid  is  not  easily  altered  and  does  not 
adjust  itself  to  the  tension.  We  can  therefore  only 
conclude  by  various  inferences,  which  cannot  be  gone  into 
lure,  that  such  tensious  do  actually  exist,  and  the 
evidence  on  this  point  is  quite  conclusive.  The  question 
then  arises,  whether  the  surface  energy  at  the  boundary 
of  a  solid  is  a  constant  for  a  given  system,  or  whether  by 
some  means  it  can  assume  a  minimum  value.  Since  it  is 
the  product  of  the  surface  tension  into  the  surface,  it  can. 
of  course,  be  reduced  by  reducing  either  factor  ;  in  tin- 
case  of  a  solid  the  second  one,  the  surface,  is  fixed,  and 
any  reduction  \\hich  may  occur  must  be  due  to  a  decrease 
in  the  first  factor,  the  surface  tension.  It  is  at  least  a 
reasonable  assumption  that  a  change  in  concentration  at  the 
boundary  surface  may  be  accompanied  by  such  a  reduction  ;  if 
this  assumption  is  correct,  such  a  change  ought  to  take 
place,  and  this  is  what  actually  happens. 

We  have  thus  come  to  the  conclusion  that,  if  in  a 
two-phase  system  a  change  in  the  concentration  of  the 
liquid  or  gaseous  phase  will  lead  to  a  decrease  of  surface 
energy,  this  change  will  occur ;  to  prove  this  view  we 
have  to  investigate  whether  in  all  cases  where  such 
changes  appear  there  is  a  diminution  of  surface  energy. 
A  change  of  concentration  can  occur  in  a  gas,  if  it  is 
compressed  at  the  boundaiy  surface,  or  in  a  solution,  it 
the  dissolved  substance  is  accumulated  at  the  surface. 
Now  we  know  that  charcoal,  for  instance,  does  condense 
gases  on  its  surface,  or  takes  colouring  matter  out  of 
solution,  but,  as  alreadjr  pointed  out,  we  have  no  means  of 


ADSORPTION   ON   SURFACE   LIQUID-GAS.  71 

measuring  the  surface  tension.  We  must  accordingly  get 
evidence  by  studying  similar  phenomena  under  conditions 
which  permit  such  measurements.,  and  this  is  possible  by 
investigating  the  behaviour  of  a  mercury  surface  against 
gases.  The  subjoined  table  gives  the  surface  tension  of 
mercury  against  vacuum  (i.e.,  its  own  vapour  only)  and 
various  gases,  one  value  being  taken  immediately  after 
formation  of  the  surface,  the  other  after  one  hour. 

Atmosphere  <r  Ifresh  surface)     <r  (after  one  hour) 

Vacuum  (15°)  •••           ...  436  436 

Hydrogen  (21")  ...  470  434 

Oxygen  (25°)       478  432 

Nitrogen  (16°)     4S9  438 

Carbon  Dioxide  (19  )       ...  480  436 

Dry  Air  (17°)        476  429 

Moist  Air  (17°) 481  429 

It  will  be  seen  that  the  two  values  measured  in 
vacuo  are  the  same,,  that  is,  no  change  has  taken  place 
after  one  hour.  In  all  the  gases,  however,  the  final  value 
of  the  surface  tension  is  considerably  lower  than  the 
initial  one,  and  this  change  is  accompanied  by  the 
condensation  of  gas  on  the  mercury  surface.  The  rate  of 
change  is  characteristic  for  each  gas,  and  curves  plotted 
with  the  times  and  tensions  as  co-ordinates  agree  very  well 
with  corresponding  concentration-time  curves  obtained  in 
the  case  of  solid  adsorbent  materials.  As  the  decrease 
in  the  surface  tension  mercury -gas  is  accompanied  by 
compression  of  the  latter,  we  must  at  once  conclude  that 
the  surface  tension  of  mercury  should  be  lowered  with 
rising  gas  pressure — a  conclusion  which  has  been  experi- 
mentally verified. 


72  ADSORPTION   IN   SURFACE  OF  SOLUTIONS. 

The  case  discussed  furnishes  us  with  an  instance  in 
which  the  concentration — or,  in  other  words,  the  pressure 
— of  a  gas  has  been  altered  at  the  surface  of  a  liquid,  with 
a  change  in  surface  energy,  and  we  conclude  that  the 
same  conditions  hold  good  at  the  boundary  sol  id -gas. 
We  may  now  consider  an  instance  of  the  surface  liquid- 
gas,  in  which  the  change  of  concentration  takes  place  in 
the  liquid  phase,  with  the  object  of  once  more  verifying 
that  it  is  accompanied  by  a  reduction  of  surface  tension. 
We  already  are  familiar  with  one  simple  criterion  of 
lowered  surface  tension  :  froth  formation.  If  we  there- 
fore take  a  solution  exhibiting  this  characteristic  and 
produce  the  largest  possible  surface,  by  making  a  froth, 
the  latter  ought  to  contain  the  dissolved  substance  in 
greater  concentration.  This  reasoning  has  also  been 
verified  experimentally  by  various  observers,  especially  by 
Miss  Benson  in  the  case  of  solutions  of  amyl  alcohol  in 
water,  which  froth  copiously.  Air  is  drawn  through  the 
solution,  which  carries  the  froth  formed  over  into  a  second 
vessel ;  this  froth  and  the  bulk  are  then  analysed 
separately. 

An  excess  of  about  5*  per  cent  of  alcohol  is  found  in 
the  froth,  which  result  again  confirms  our  reasoning. 

We  have  thus  some  direct  evidence  to  support  the 
view  that  the  changes  of  concentration,  classed  together 
as  adsorption;  on  a  surface  are  due  to  the  tendency  of  the 
surface  energy  to  attain  a  minimum  value,  and  that  they 
occur  if  an  increased  concentration  leads  to  a  reduced  surface 
tension.  Our  whole  knowledge  of  the  matter,  however,  is 
not  based  exclusively  on  such  reasoning,  which,  as  far  as 
the  solid  surface  is  concerned,  rests  on  the  uncertain 
ground  of  analogy,  but  the  principal  proposition  has  been 
proved  by  thermodynamical  methods  by  Willard  Gibbs. 


GIBBS'S   FORMULA.  73 

He  arrived  at  the  following  famous  formula  :  — 


RT  dC 
in  which  the  symbols  mean  : 

U  excess  of  substance  in  surface  layer 

C  concentration  in  bulk  of  liquid 

<r  surface  tension 

R  the  gas  constant 

T  the  absolute  temperature. 

The  formula  contains  the  differential  coefficient  of 
the  function  connecting  surface  tension  and  concentration, 
which  is,  of  course,  positive  if  both  change  in  the  same 
sense,  and  negative  if  they  change  in  opposite  senses. 
This,  in  conjunction  with  the  minus  sign  on  the  right 
hand  of  the  equation,  shows  at  once  that  there  will  be  a 
negative  excess,  i.e.,  a  diminished  concentration  in  the 
surface,  if  the  surface  tension  increases  with  increasing 
concentration,  and  a  positive  excess,  i.e.,  increased  concen- 
tration in  the  surface,  if  the  surface  tension  decreases  with 
increasing  concentration.  The  latter  is  the  more  common 
case,  and  confirms  the  results  previously  arrived  at  ;  the 
former,,  however,  has  also  been  experimentally  observed, 
and  is  known  as  negative  adsorption. 


CHAPTER    IX. 

Further  conclusions  can  be  drawn  from  the  formula. 
As  the  absolute  temperature  appears  in  the  denominator, 
the  excess  in  the  surface,  or,  in  other  words,  the  amount 
adsorbed — whether  positive  or  negative — varies  inversely 
with  the  temperature,  and  decreases  as  the  latter  rises. 
It  also  follows  that  a  small  amount  of  dissolved  substance 
can  lower  the  surface  tension  greatly  but  can  only  increase  it 
slightly. 

This  somewhat  surprising  statement  becomes 
intelligible  when  we  remember  that  surface  tension 
manifests  itself  only  in  the  surface  layer  and  depends 
purely  on  the  composition  of  the  latter.  If  a  dissolved 
substance  in  increasing  concentration  increases  the  surface 
tension,  the  formula  tells  us  that  its  concentration  in  the 
surface  layer  is  less  than  in  the  bulk  of  the  liquid,  and  its 
effect  is  thus  counteracted  to  some  extent.  On  the  other 
hand,  if  in  increased  concentration  it  reduces  the  surface 
tension,  it  accumulates  in  the  surface  layer,  thus 
enhancing  its  effect.  As  a  matter  of  experience,  minute 
amounts  of  accidental  impurities  often  reduce  the  observed 
values  of  surface  tensions  considerably,  while  increases 
owing  to  small  amounts  of  unintentional  admixtures  are 
not  met  with. 

When  more  than  one  substance  is  present  in  a 
solution,  the  process  becomes  necessarily  complicated, 
but  one  or  two  points  majr  be  discussed  briefly.  It  is 
quite  possible  that  the  various  substances  ma}'  not  be 
adsorbed  to  the  same  extent,  in  which  case  one  or  the 


VARIors  ADSORBENTS.  i  .) 

other  may  be  removed  selectively,  as,  e.g.,  the  colouring 
matter  from  sugar  solutions.  Where  a  single  compound 
is  dissolved  in  a  dissociating  solvent,  the  ions  may 
likewise  not  be  adsorbed  equally,,  and  the  solution, 
originally  neutral,  may  be  acid  or  alkaline  after  adsorption  ; 
this  also  has  been  found  to  be  the  case,  for  instance,  in 
van  Bemmeleii's  experiments  on  the  adsorption  of 
potassium  sulphate  by  gels,  when  the  remaining  solution 
was  found  acid,  i.e.,  the  K*  ion  was  adsorbed  to  a  greater 
extent  than  the  SO4  ion. 

Although  the  formula  does  not  include  this  term,  it 
is  obvious  that  the  amount  adsorbed,  other  things  being 
equal,  is  proportional  to  the  active  surface — all  substances 
indeed,  which  are  employed  as  adsorbents  have  very  large 
surfaces.  Charcoal  retains  the  cellular  structure  of  the 
raw  material  ;  kieselguhr  consists  of  very  fine  and 
•extremely  complicated  silicious  skeletons  of  diatoms, 
while  the  gels,  as  we  know,  also  possess  a  network  of 
cellular  structure  with  enormous  surface.  A  material 
which  has  recently  come  into  some  prominence,  especially 
through  the  work  of  Wislicenus,  is  the  substance  called 
in  German  "  gewachsene  Tonerde,"  i.e.,  u  sprouted 
alumina."  This  is  an  aluminium  hydroxide,  obtained 
by  the  oxidation  of  aluminium  in  presence  of  moisture 
and  of  extremely  small  quantities  of  mercury.  It  has  a 
typical  gel  structure,  and  that  its  adsorbent  effect  is  due 
to  the  latter  is  shown  by  the  action  remaining  unaltered 
when  the  substance  is  dehydrated  at  red  heat. 

While  we  have  thus  a  number  of  qualitative  data 
regarding  the  phenomenon,  one  question  is  still  open  : 
whether  it  proceeds  to  any  definite  end  point  or 
equilibrium,  and,  if  so,  in  what  terms  this  can  be 
expressed  mathematically.  These  points  are  nowr  settled 


76  ADSORPTION  EQUILIBRIUM. 

by  an  enormous  amount  of  material  collected  by  various 
observers.  One  of  the  first  experiments  dealing  with 
this  problem  was  made  by  Wilhelm  Ostwald,  who  placed 
a  quantity  of  charcoal  in  dilute  hydrochloric  acid,  and 
after  a  certain  time  determined  the  concentration  of  the 
latter.  If  then  a  portion  of  either  the  charcoal  or  of  the 
acid  was  removed,  no  further  change  took  place,  which 
tends  to  show  that  an  equilibrium  between  the  concentra- 
tions —  on  the  surface  and  in  the  bulk  of  the  acid  —  has 
been  attained.  Further  decisive  experiments  are  due  to- 
Freundlich,  who  placed  charcoal  in  solutions  of  acetic  and 
of  benzoic  acids  of  known  strengths  and  determined  the 
amount  adsorbed.  The  same  amount  of  charcoal  wrere 
then  placed  into  half  the  volumes  of  acid  ot  double  the 
strength  used  in  the  first  experiments  and  after  a  time 
an  equal  volume  of  solvent  was  added,  bringing  the  total 
volumes  to  those  used  in  the  first  instance.  If  there  is  a 
definite  equilibrium  between  the  adsorbed  quantities  and 
the  end  concentrations,  the  final  concentration  in  the 
second  experiment  should  be  the  same  as  in  the  first,  and 
Freundlich  in  fact  found  this  to  be  the  case. 

Mathematical  investigation  confirmed  by  an 
enormous  amount  of  experimental  work  with  a  variety  ot 
adsorbents,  solvents,  and  dissolved  substances  has 
established  a  definite  relation  between  the  quantity  of 
adsorbent  m,  the  quantity  adsorbed  y,  and  the  end  or 
equilibrium  concentration  c  in  the  liquid  after  adsorption, 
which  takes  the  following  form  :  — 


in    which    a    and    n    are    constants    depending     on    the 
nature  of  the  solutions  and   the  adsorbent.        The  curve 


THE   ADSORPTION  ISOTHERM.  77 

corresponding    to    the  above   equation    is   known   as    the 
adsorption    isotherm,"    and    it    is   obvious    that   it    is   a 
parabolic     curve ;     for    n  ==  2,   it   actually   becomes    the 
ordinary  conic  parabola. 

It  may  be  pointed  out  here  that  the  formula  is  very 
frequently,  but  quite  erroneously,  spoken  of  as  an 
exponential"  one.  An  exponential  expression  is  one 
containing  one  of  the  variables — in  the  present  case  these 
are  y  and  c — as  exponent,  whereas  the  exponent  in  the 
equation  of  the  adsorption  isotherm  is  a  constant. 
It  is  an  interesting  fact  that  this  constant  varies 
within  comparatively  narrow  limits  for  the  most  widely 
different  substances,  viz.,  roughly  speaking,  between 
n  =  2  and  n  =  !O. 

The  principal  deduction  from  the  equation  is  obvious : 
the  amount  adsorbed,  other  things  being  constant, 
increases  much  more  slowly  than  the  concentration  of  the 
solution.  This  becomes  quite  clear  by  choosing  a  simple 
example  in  figures,  say  n  —  2,  and  m  and  a  =  *• 
(the  latter  is  simplification  always  admissable  in  any  one 
series  of  experiments).  In  this  case 

<?2,  or,  in  a  more  familiar  form,  y  =  \]  c. 

If  we  therefore  take  c,  the  end  or  equilibrium  con- 
centration successi ve\y  equal  to  1,  4,  9,  16,  we  find  the 
adsorbed  amounts  to  be  the  square  roots  of  these  numbers, 
i.e.,  \,  2,  3,  4.  This  means  that,  if  double  o^  treble  the 
amount  of  substance  is  to  be  absorbed,  the  remaining 
solution  must  be  four  or  nine  times  as  concentrated  as  for 
unit  adsorption.  As  the  initial  concentration  of  each 
solution,  before  adsorption,  is  evidently  y  +  c,  we  find 
that  the  initial  concentrations  corresponding  to  the 
adsorbed  amounts  chosen  must  be  2,  6,  12  and  20. 


'8 


VALUES  OF   EXPONENT  IN   ADSORPTION   FORMULA. 


Accordingly,  in  the  example  chosen,  the  initial  concentra- 
tion must  be  increased  ten  times  if  the  adsorbed  amount 
is  to  be  four  times  as  great. 

The    adsorption    isotherms    and    the    exponents    1_ 

// 

have  been  determined  for  many  substances  and  solvents. 
The  table  below  shows  a  number  of  determinations  by 
Freundlich. 


Adsorbent.                 i      Solvent. 

Substance  Dissolved. 

i 

n 

Blood  charcoal          ...       Water 

Formic  acid 

0-451 

,, 

Acetic  acid 

0*425 

... 

Benzoic  acid 

0*338 

( 

Picric  acid 

()'24() 

( 

: 

Chlorine 

(V2()7 

t 

... 

Bromine              ...          0'340 

Ben/ol 

Ben/oic  acid      ...          0'416 

t 

...  j 

Picric  acid          ...          (V302 

Water 

Patent  Blue 

0'190 

Wool           ...             ...  I 

Patent  Blue 

0  159 

Silk 

1 

Patent  Blue 

0-163 

The  table  shows  clearly  the  limits  between  which  the 
exponent  varies,  and  a  further  point  of  interest  is  raised 
by  the  behaviour  of  the  same  solution — Patent  Blue  in 
water — towards  three  adsorbents  as  different  as  charcoal, 
wool,  and  silk.  The  value  of  the  exponent  does  not  differ 
greatly  in  three  cases,  and  it  has  been  very  generally 
observed  that  the  influence  of  the  absorbent  is  very  slight 
compared  with  that  of  the  other  factors.  Various 
adsorbents  have  been  investigated,  but  no  quantitative 
relation  has  been  established,  and  the  difficulty  of  doing 
so  becomes  obvious  when  we  remember  that  we  have  no 
means  of  determining  and  comparing  the  active  surfaces 


EFFECT  OF  ADSORBENT.  79 

of  substances  like  charcoal  and  fibres.  It  is,  however, 
fairly  well  established  that  the  order  in  which  various 
dissolved  substances  are  adsorbed  is  the  same  for  different 
adsorbents  ;  if  a  substance  A  is  more  strongly  adsorbed 
than  another  B,  and  the  latter  more  than  C,  by  charcoal, 
the  same  order  will  hold  good  for  other  adsorbing 
materials,  although  the  numerical  ratios  may  be  altered. 


FIG.  13. — TYPICAL  ADSORPTION  ISOTHERMS. 


Another  question  of  importance  has  not  been  touched 
on  so  far,,  that  is  the  effect  of  the  solvent,  in  all  cases 
where  a  substance  is  soluble  in  more  than  one  liquid.  It 
is  well  known,  even  from  general  experience,  that  the 
same  substance  is  not  adsorbed  equally  out  of  solutions  in 
different  solvents,  and  that  adsorption  is  much  slighter  in 
organic  solvents  than  in -water.  Thus  Freundlich  gives 


80  EFFECT    OF    SOLVENT. 

the  following  figures  for  the  adsorption  of  benzole  acid 
out  of  solutions  of  equal  strength  in  : — 

Water  4' 27 

Benzol  ...  0'55 

Ether  ...  . 0'30 

Acetone  ...  ...          ...  0'3 

This  peculiarity  of  organic  solvents  is  practically  used 
for  removing  substances  adsorbed  out  of  aqueous  solutions, 
A  dilute  aqueous  solution  of  a  dye,  like  crystal  violet,  can 
be  completely  decolourised  by  charcoal.  If  the  latter  is 
then  placed  in  alcohol,  the  adsorption  from  \vhich  is  much 
lower,  the  surface  concentration  of  the  dye  on  the  char- 
coal is  an  excess  of  that  which  would  establish 
equilibrium,  and  a  large  amount  of  it  therefore  goes  into 
solution. 

Two  typical  adsorption  isotherms  are  shown  in  Fig.  1 3   . 
They  are  also  due  to  Freundlich,  and  striking^  illustrate 
the  parabolic  character  ot  the  curve. 

We  are  now  in  a  position  to  explain  Freundlich's 
views  of  the  precipitation  ol  suspensoids  by  electrolytes, 
to  which  reference  has  been  made.  It  will  be  re- 
membered that  trivalent  ions  act  in  much  lower  concentra- 
tions than  divalent,  and  these  again  in  lower  concentrations 
than  monovalent  ions,  but  that  the  amounts  actually 
found  in  the  precipitate  are  equivalent.  This  means,  that 
for  each  trivalent  ion  two  divalent  or  three  monovalent 
ions  must  be  provided,  as  these  "amounts  carry  the  same 
electrical  charges.  Freundlich  suggests  that  the  process, 
or  at  least  the  first  step  in  it,  is  an  adsorption.  If  the 
different  ions  are — as  there  is  reason  to  believe — adsorbed 
at  about  equal  rates,  we  can  refer  the  process  to  one  and 
the  same  adsorption  isotherm  for  all  three.  By  this  mean 


ELECTROLYTE  COAGULATION  ;  FREUNDLICH's  THEORY.         81 

we  can  find  the  concentrations  which  are  necessary  to 
allow  the  corresponding  amounts  to  be  adsorbed.  In 
other  words,  we  draw  three  ordinates  of  the  isotherm 
which  are  •  in  the  ratio  1 :  j>  :  ,S,  and  the  corresponding 
abscissae  give  the  concentrations.  By  doing  this  on  one 
of  the  isotherms  in  Fig.  13,  it  will  be  seen  at  once  how 
much  greater  the  abscissa  becomes  for  the  ordinate  4, 
representing  the  divalent  ion,  than  for  the  ordinate  \, 
corresponding  to  the  trivalent  ion,  and  that  the  abscissa 
of  the  ordinate  3,  which  represents  the  monovalent  ion,  is 
again  very  much  larger  than  that  of  the  ordinate  2. 

The  different  rate  of  adsorption  of  substances  present 
in  a  solution  can  be  strikingly  demonstrated,  and  can  be 
utilised  for  proving  their  presence  in  extremely  minute 
quantities  by  allowing  the  solution  to  rise  in  long  strips  of 
filter  paper.  While  this  takes  place,  the  dissolved 
substances  are  adsorbed  by  the  fibre,  so  that  beyond  a 
certain  height  the  liquid  in  the  paper  consists  of  pure 
solvent  only.  Different  substances  generally  rise  to 
different  heights,  and  can  be  identified  by  their  colour,  or 
if  colourless,  by  appropriate  reactions.  The  process  can 
be  demonstrated,  for  instance,  with  a  very  dilute  solution 
ot  turmeric  and  picric  acid,  which  is  allowed  to  rise  in  a 
strip  of  filter  paper  about  1 2  ins.  long.  The  strip  is 
stained  yellow,  but  if  it  is.  then  exposed  to  ammonia,  only 
the  lower  portion  turns  brown,  showing  that  the  turmeric 
has  not  risen  as  far  as  the  picric  acid.  The  method, 
which  deserves  to  be  more  widely  known  than  appears  to 
be  the  case,  may  be  used  for  showing  the  presence  of 
colouring  matter  or  preservatives  in  articles  of  con- 
sumption, and  for  many  similar  purposes.  It  has  been 
developed— under  the  title  of  "Capillary  analysis  "- 
principally  by  F.  Goppelsroeder,  of  Basle,  who  has 


82  ADSORPTION  AND  CHEMICAL  COMBINATION. 

demonstrated  its  extreme  sensitiveness  in  favourable 
cases. 

It  is  obvious  that  in  all  instances  in  which  reactions 
take  place  in  the  presence,  or  lead  to  the  formation,  of 
finely  divided  solid  matter,  adsorption  is  possible,  and  may 
account  for  changes  in  concentration  or  losses.  It  offers 
thus  a  somewhat  eas}r  explanation  of  such  phenomena, 
which,  however,  ought  not  to  be  taken  as  established 
without  investigation,  that  is,  measurements  and  the 
plotting  of  a  curve,  which  must  have  the  character  of  the 
adsorption  isotherm,  to  establish  this  interpretation  of 
what  occurs.  If  the  curve  obtained  is  of  a  different  type, 
it  shows  that  adsorption,  if  it  takes  place  at  all,  is- 
accompanied  by  some  other  process.  In  this  connection 
it  should  also  be  borne  in  mind  that,  even  if  chemical 
combination  between  the  adsorbent  and  the  adsorbed 
substance  is  possible,  the  two  processes  need  not  occur 
simultaneously.  This  point  is  strikingly  illustrated  in  an 
experiment  by  Bayliss,  in  which  the  blue  Congo  red  acid — 
liberated  from  the  dyestuff,  which  is  a  sodium  salt,  by  acid 
— is  adsorbed  by  aluminum  hydroxide.  The  latter  is 
stained  blue,  although  all  the  salts  of  the  acid  are  red? 
which  shows  that  adsorption  has  taken  place  without 
chemical  combination.  The  latter,  however,  occurs  on 
warming,  when  the  colour  of  the  hydroxide  changes  from 
blue  to  red. 

.  .  A  process,  which  in  a  way  may  be  looked  upon  as 
the  converse  of  adsorption,  is  the  extraction  of  a  substance 
containing  an  admixture,  with  a  solvent  in  which  the 
latter  only  is  soluble.  It  is  evident  that,  if  there  is 
simply  mechanical  mixture,  and  if  a  sufficient  quantity  of 
solvent  is  employed,  the  whole  or  the  soluble  matter  will 
be  extracted  by  the  first  lot  of  solvent.  It  is  equally 


EXTRACTION    AS    IXVEKSIOX    (»K   ADSORPTION.  83 

obvious  that  this  cannot  be  the  case  if  the  second 
substance  is  adsorbed  by  the  first :  in  that  event  the  first 
lot  of  solvent  will,  indeed,  remove  a  large  fraction  of  the 
soluble  matter  :  but  as  much  of  it  as  establishes  equilibrium 
under  the  given  conditions  will  be  retained.  A  second 
lot  of  solvent  will  again  remove  a — much  smaller — 
quantity,  and  so  on.  If  a  curve  is  plotted  with  the, 
preferably  equal,  volumes  of  solvent  as  abscissae,  and  the 
amounts  of  soluble  matter  still  retained  as  ordinates, 
curves  of  a  hyperbolic  type  are  obtained,  and  it  is  quite 
easy  from  these  to  construct  the  adsorption  isotherm  for 
the  system  under  examination,  as  will  be  clear  from  an 
actual  example.  Rubber,  as  is  well  known,  contains 
varying  amounts  of  "resin,"  i.e.,  of  substances  soluble  in 
acetone,  and  the  results  of  extraction  with  successive 
equal  portions  of  that  solvent  are  shown  in  Fig  14,  taken 
from  an  investigation  by  D.  Spence  and  J.  H.  Scott, 
published  in  the  Kol/oid-/eitschrift."  The  amounts  of 
resin  still  retained  after  each  extraction  are  plotted 
as  ordinates  of  the  curve  in  full  line  at  equal  distances 
apart.  It  will  be  noticed  at  once  that  the  first  lot  of 
solvent  extracts  a  very  large  portion  of  the  total  resin 
contents,  as  shown  by  the  length  ab  on  the  ordinate  ac. 
Simililarly,  the  portion  extracted  by  the  second  lot  of 
acetone  is  given  by  the  —  much  smaller  —  length 
ft  b'  on  a  c  .  The  curve  resembles  a  hyperbola  and 
it  can  easily  be  shown  that  the  whole  process  is  an 
inverted  adsorption,  both  by  analytical  and  by  graphical 
methods.  .To  adopt  the  latter,  we  consider  the  two 
portions  of  the  ordinate  ac  ;  the  part  ab  has  gone  into 
solution,  while  be  is  retained  by  the  rubber.  The  latter 
therefore  represents  the  amount  adsorbed  which  is  in 
equilibrium  with  the  concentration  in  the  solvent 


EXTRACTION   AND   ADSORPTION   CURVES. 


produced  by  dissolving  the  quantity  ab.  If,  accordingly, 
\ve  plot  the  lengths  ab  a  //,  —  -  as  abscissae,  and  the 
lengths  be,  be-  —  as  ordinates  we  obtain  a  curve 
which  must  have  the  character  of  the  adsorption  isotherm 
if  the  extracted  matter  is  really  retained  by  adsorption  on 
the  insoluble  portion.  This  curve  is  plotted  in  dotted 


FIG.  14. 
line,  and  it  is  obvious  that  it  is  of  the  familiar  parabolic 

type. 

(in  the  actual  plotting  of  the  dotted  curve  the 
ordinates  have  been  doubled,,  to  obtain  a  larger  scale.) 

We  have  so  far  considered  only  phenomena  in  which 
the  change  in  surface  energy  has  been  held  to  be  the 


ELECTRIC  ADSORPTION.  85 

determining  factor,  and  have  disregarded  the  fact,  with 
which  we  are  already  familiar,  that  boundary  surfaces  are 
generally  the  seats  of  electric  charges.  It  is  more  than 
probable  that  these  may  affect  adsorption,  and  there  are 
some  striking  phenomena  in  which  the  electric  factors 
appear  to  play  the  most  important  or  indeed  an  exclusive 
part.  If,  for  instance,  a  ferric  hydroxide  solution  is 
passed  through  a  column  of  sand — carefully  purified — the 
hydroxide  is  completely  retained,  and  only  clear  water 
leaves  the  end  of  the  column  for  a  time.  The  same  thing 
occurs  with  a  solution  of  Night  blue,  as  has  been  shown 
by  Dreaper  and  Davis.  In  both  cases  the  sand  is 
capable  of  retaining  only  a  quite  definite  quantity  :  when 
this  has  been  reached,  the  liquid  passes  through  unaltered 
Both  ferric  hydroxide  and  Night  blue  belong  to  the,  not 
very  numerous,  class  of  positive  colloids,  while  silica,  like 
most  substances,  assumes  a  negative  charge  in  contact 
with  water.  It  is,  therefore,  reasonable  to  assume  that 
the  positive  colloidal  particles  are  discharged  and  retained 
by  the  negatively  charged  sand  grains.  Night  blue  is 
retained  with  such  tenacity  even  by  a  smooth  glass 
surface  that  vessels  which  have  contained  the  solution 
cannot  be  washed  clean  with  water  alone.  The 
phenomenon  does  not  occur  in  an  alcoholic  solution  of  the 
dye,  and,  if  the  latter  has  been  adsorbed  on  sand  from 
an  aqueous  solution  it  can  be  removed  by  subsequent 
washing  with  alcohol. 

It  must  be  noted  that  the  clear  liquid  which  passes 
out  of  the  column  of  sand  is  no  longer,  like  the  dye 
solution,  neutral,  but  acid,  i.e.,  dissociation  has  taken 
place.  The  adsorption  of  highly  dissociated  substances 
is  a  subject  to  which  passing  reference  has  already  been 
made,  but  our  knowledge  in  this  respect  is  still  very 


#6  ADSORPTION   OF   IONS. 

incomplete.  Cases  of  both  positive  and  negative  adsorp- 
tion are  known,,  and  also  numerous  instances  in  which  the 
-anioii  and  the  cation  are  not  adsorbed  in  equivalent 
quantities,  that  is,  the  solutions  after  adsorption  show  an 
excess  of  either,  generally  the  anioii.  This,  as  already 
mentioned,  is  the  case  with  potassium  sulphate  in  van 
Itemmelen's  experiments,  also  with  salts  of  aniline  and 
with  many  dyestuffs.  The  whole  subject  is  in  urgent 
need  of  much  further  investigation. 

The  general  importance  of  adsorption  hardly  needs 
insisting  on.  Its  connection  with,  and  special  importance 
in  the  study  of  colloids  is  also  obvious  ;  since  all  the 
systems  dealt  with  under  this  head  posses  very  large 
surfaces,  adsorption  is  an  essential,  if  sometimes  very 
obscure  factor  of  the  whole  complex  of  phenomenon  to  be 
observed.  Thus  adsorption  undoubtedly  takes  place,  not 
only  in  gels,  but  also  on  the  surface  of  the  disperse  phase 
in  sols.  This  has  been  proved  directly,  by  means  of 
-conductivity  measurements,  by  Wolfgang  Ostwald,  and  by 
several  other  observers.  Many  authorities  even  hold  that 
the  electric  charge  on  the  particles  is  due  to  absorbed  ions 
.and  that  the  coagulation  by  electrolytes  belongs  to  the 
same  category,  viz.,  is  an  adsorption  phenomenon. 
Reference  to  Freundlich's  viewrs  and  his  explanation  of 
the  difference  in  effect  between  the  cations  of  different 
valency  has  already  been  made.  Without  going  into  the 
details  of  what  are  still  highly  controversial  questions, 
reference  to  them  is  necessary  to  remind  the  student  that 
adsorption  is  likely  to  play  an  important,  if  not  the 
determining,  part  in  the  formation  and  transformation  of 
all  disperse  systems. 


CHAPTER  X. 

In  the  preceding  chapters  the  reader  has  been 
furnished  with  a  description  —necessarily  brief,  but  not 
omitting  any  feature  of  general  importance— of  the 
principal  properties  of  disperse  systems.  It  now  becomes 
desirable  to  examine  this  whole  mass  of  material  with  a 
view  to  finding,  if  possible,  some  general  factors 
connecting  the  very  various  and  striking  phenomena. 

The  most  general  factor  of  this  kind  is  the  boundary 
surface  between  the  phases,  whatever  their  nature  or 
state  of  aggregation.  Although  the  increase  in  surface 
with  the  sub-division  of  a  given  mass  into  smaller  and 
smaller  particles  is  obvious,  it  is  desirable  to  examine  it 
mimericallj'.  If  we  start  with  the  unit  volume,  one  cubic 
centimetre,,  in  the  shape  of  a  cube,  the  surface  is  6  square 
centimetres.  If  we  now  subdivide  the  mass  into  cubes 
having  an  edge  of  one  tenth  of  the  original  dimension, 
vi/.,  one  mm,  we  obtain  1000  cubes,  each  of  which  has  a 
surface  of  6  sq.  mm,  so  that  the  aggregate  surface  of  the 
subdivided  mass  is  now  60  sq.  cm.  or  ten  times  the 
original  surface.  It  will  easily  be  found  that  on  further 
subdivision  the  aggregate  surface  grows  in  inverse  ratio 
with  the  linear  dimension.  If  we  subdivide  the  unit 
mass  into  cubes  of  ultra-microscopic  dimension,  say  with 
an  edge  of  1  x  10"G  cm.  or  Wpp,  the  aggregate  surface 
becomes  6  X  10(>  sq.  cm.  or  60  square  metres.  This 
surface  which  the  unit  volume  assumes  through 


88  SYSTEMS  OF  TWO  PHASES  ;    STATE  OF  DISPERSE  PHASE. 

subdivision  in  any  given  system  has  been  called  by  Wo. 
Ostwald  the  specific  surface  of  the  system.  Disperse 
systems  begin  to  show  the  characteristic  properties  which 
we  have  described  when  the  specific  surface  reaches  the 
order  of  105  sq.  cm. 

We  have  been  led  to  distinguish  between  systems  in 
which  the  disperse  phase  consists  of  solid,  or,  more 
correctly,  undeformable  particles,,  and  those  in  which  the 
disperse  as  well  as  the  continuous  phase  are  liquid,  that 
is,  the  disperse  particles  are  deformable.  In  examining 
the  former,  we  have  found  that  there  is  a  steady 
transition  from  suspensions  to  "  suspensoids,"  due  to  the 
decreasing  size  of  the  particles  and  the  consequent  low 
velocity  of  settlement,  increasing  Brownian  movement 
and  increasing  importance  of  the  electric  factors,  which 
latter  follows  from  the  great  increase  of  specific  surface. 
The  influence  of  the  electric  charge,  although  obscure 
both  in  its  origin  and  its  effect,  on  the  stability  of  these 
systems  has  also  received  discussion. 

In  considering  systems  of  two  liquid  phases,  we  have 
found  that  their  chief  mechanical  difference  from 
.suspensiods  lies  in  the  possibility  of  deformation,  which 
makes  it  possible  to  have  any  phase  ratio.  We  found 
that  this  phase  ratio,  or  rather  a  large  volume  of  disperse 
phase  with  a  small  volume  of  continuous  phase,  is  essential 
for  the  manifestation  of  high  viscosity.  These  points 
refer  both  to  emulsions  and  to  emulsoids,  but  a  further 
peculiarity  of  the  latter  now  deserves  insisting  on  :  the 
ready  displacement  of  solvent  from  one  phase  into  the 
other.  We  know  that  the  disperse  phase  of  the  emulsoid 
consists  of  some  form  of  aggregates  containing  large 
amounts  of  the  solvent,  and  the  striking  changes  in 
viscosity,  and  the  sol-gel  transformations  under  the 


ADSORPTION,   GENERALITY   OF   DISPERSE  SYSTEMS.  89 

influence  of  temperature,  dissolved  substances,  etc., 
become  referable  to  such  a  displacement  of  the  solvent 
from  one  phase  into  the  other. 

In  the  emulsoid  gels,  such  as  silicic  acid,  gelatine 
and  agar  gel,  we  have  finally  become  familiar  with 
systems  in  which  the  continuous  phase  is  solid — or,  again 
to  avoid  certain  difficulties — less  deformable,  and  forms 
a  skeleton  or  network  filled  with  liquid.  In  these 
systems  again  the  effect  of  surface  becomes  very  marked, 
as  some  of  their  most  characteristic  elastic  and  optical 
properties  are  due  to  it. 

We  have  finally  studied  in  some  detail  the  important 
phenomenon  of  adsorption  or  increased  concentration  on 
boundary  surfaces.  We  have  found  that  this  occurs 
inevitably  in  all  systems  with  large  surfaces,  and  that 
it  depends  on  two  forms  of  energy  inseparable  from  such 
surfaces,  namely,  electric  and  surface  energy.  Either  of 
these  factors  may  predominate,  or  both  may  be  active,  in 
which  case  the  phenomena  become  necessarily  complicated 
as  in  the  case  of  dissociated  substances. 

While  we  have  thus  gained  some  knowledge  of  the 
most  important  and  striking  properties  of  disperse  systems 
there  is  one  question  of  enormous  theoretical  interest  to 
which  no  reference  has  so  far  been  made.  This  question 
may  be  put  as  follows  :  what  factors  determine  in  any 
given  case  whether  a  highly  disperse  system  results  at 
all  ?  The  problem  has  been  put  in  this  form  by  von 
Weimarii,  who  has  also  in  a  series  ot  researches  of  great 
"brilliance  provided  a  complete  answer.  To  summarize 
this  briefly,  certain  relations  must  exist  between  the 
solubility  of  the  disperse  phase  in  the  dispersion  medium 
and  the  supersaturation  existing  at  the  moment  of  its 
production,  so  that  the  growth  of  particles  is  limited  to 


90  THEORY   OF   BROWNIAN  MOVEMENT. 

the  small  dimensions  required  in  a  stable  disperse  systems. 
If  these  factors  are  known  it  is  possible  to  predetermine 
the  concentration  of  two  reacting  solutions  so  that  the 
product  of  reaction  will  form  a  sol.  Working  on  this 
plan  v.  Weimarn  and  his  pupils  have  in  fact  succeeded  in 
preparing  some  hundreds  of  suspensoid  sols  of  a  very 
great  variety  of  substances.  Readers  interested  in  the 
subject,,  which  will  eventually  be  of  fundamental 
importance  not  only  for  our  present  subject,  but  for  our 
knowledge  of  certain  general  properties  of  matter,  must 
be  referred  to  v.  Weimarn's  numerous  monographs. 

It  now  remains  to  devote  a  few  words  to  applications 
of  colloidal  science,  both  theoretical  and  practical.  As 
regards  the  former,  it  can  boast  of  one  brilliant  success  : 
the  study  of  the  Brownian  movement  by  Svedberg, 
Einstein,  v.  Smoluchowski  and  Perrin,  which  has  afforded 
the  most  striking  and  convincing  demonstration  of  the 
real  existence  of  molecules.  Less  dazzling,  but  full  of 
promise,  are  the  numerous  physiological  investigations 
based  on  the  properties  of  colloids  which  gradually  tend 
to  elucidate  how  many  fundamental  phenomena,  such  as 
muscle  contraction,  become  explicable  as  processes  of 
water  displacement  due  to  alterations  in  the  reaction  of 
the  tissue.  The  study  of  adsorption  compounds  is 
beginning  to  clear  up  a  great  number  of  debated  questions 
in  many  different  fields,  such  as  the  properties  of  arable 
soils  at  one  end  of  the  scale,  and — what  is  one  of  the 
oldest  and  most  famous  debatable  points  in  that  branch 
of  science — the  nature  of  the  latent  image  in  the 
sensitive  film  of  the  photographic  plate,  at  the  other. 

As  regards  the  practical  applications,  it  may  perhaps 
be  well  to  remind  the  reader,  on  one  hand,  of  the  youth 
of  the  whole  discipline  discussed  here,  and,  on  the  other 


APPLICATION   OF  COLLOIDAL  SCIENCE.  91 

of  the  twofold  way  in  which  the  development  of  a  new 
branch  of  science  may  bear  on  industries  and  arts.  It 
may,  of  course,  lead  directly  to  new  processes  and 
manufactures  :  an  instance  is  the  production  of  squirted 
filaments  of  the  refractory  metals  for  incandescent  lamps,, 
which  were  made  from  the  extremely  finely  divided 
metal  coagulated  from  its  sol.  The  much  more  general 
case — for  which  the  growth  of  the  chemical  industries- 
and  of  chemistry  provides  an  illustration — is  that  it 
provides  an  explanation  of  phenomena  long  known  and 
dealt  with  empirically.  In  this  direction  there  is 
evidently  a  huge  field  for  colloidal  science.,  and  the 
tilling— or  at  least  the  preliminary  weeding  and 
grubbing  of  the  ground — may  be  said  to  be  in  vigorous 
progress.  In  all  the  industries  which  deal  with  organic 
rawr  materials,  such  as  the  textile  and  d\'ing  industries, 
brewing,  tanning,  the  manufacture  of  explosive  and  other 
cellulose  derivatives,  to  name  only  a  few,  there  are 
numerous  problems  which  have  so  far  withstood  solution 
by  the  methods  of  chemistry  alone.  For  these  the  study 
of  colloids  provides,  if  not  an  answer  at  the  first  approach 
at  least  an  entire!}'  new  method  of  attack.  It  shows  us 
that  the  mere  subdivision  of  matter,  or,  in  other  words, 
the  production  of  large  surfaces,  brings  into  play  energies 
the  effects  of  which  may  be  of  the  most  varied  character, 
and  of  the  most  profound  importance.  It  teaches, 
further,  that  these  effects  may  again  be  modified 
extraordinaril}'  by  small  alterations  in  the  reaction  of  the 
medium,  alterations  which  have  no  purely  chemical 
explanation.  It  has  familiarised  us  with  the  idea  of 
adsorption  compouds,  i.e.,  combinations  in  any  ratio,  but 
perfectly  definite  in  certain  circumstances  of  temperature 
and  concentrations.  It  is,  of  course,  for  the  investigator 


92  SCOPE    FOR    RESEARCH. 

to  apply  all  these  considerations  to  his  own  problems, 
but  he  may  be  assured  that  there  still  is  much  room 
for  them  and  him — not  only,  as  the  familier  phrase, 
which  is  meant  to  stimulate,  but  succeeds  in  depressing, 
has  it  "at  the  top,"  but  even  at  the  bottom  among  the 
foundations. 


APPENDIX. 


Although  a  number  of  directions  for  experiments 
have  been  given  incidentally  in  the  preceding  chapters,  it 
seems  desirable  in  the  absence  of  any  special  work 
devoted  to  the  subject,  to  deal  somewhat  more  explicitly 
with  the  experimental  technique  of  colloidal  chemistry. 

The  methods  which  it  employs  may  be  conveniently 
divided — although  with  a  certain  amount  of  overlapping 
— into  methods  of  examination  or  identification,  and 
methods  of  preparation.  Among  the  former  the  optical 
methods  claim  the  first  place,  of  which  the  simplest  and 
the  most  sensitive  is  examination  by  the  Tyndall  cone. 
This  can  be  carried  out  by  placing  the  liquid  in  a 
rectangular  glass  cell,  one  side  of  which  is  covered  with  a 
dead  black  material,  e.g.,  black  velvet,  projecting  a 
slightly  convergent  beam  of  light  through  it  parallel 
with  the  black  ground,  and  observing  the  path  of  the 
beam  through  the  opposite  face.  If  the  cone  is  visible, 
the  presence  of  disperse  particles  is  proved  ;  unfortunately 
the  method  is  so  sensitive  that  even  freshly  distilled 
water  or  true  solutions  prepared  with  it,  show  a  perceptible 
cone.  The  preparation  and  storing  of  "optically  void" 
water  are  difficult  and  complicated  operations  beyond  the 
scope  of  this  work.  When  it  is  not  available,  a  comparison 
between  the  water  used  for  making  a  given  preparation 
and  the  latter  should  be  made,  and  should  show  a  marked 


TYNDALL  f ONE   ANALYSED. 


difference  if  the  proof  of  heterogeneity  is  to  be  conclusive. 
Tn  many  cases  the  cone  shows  a  colour  different  from  that 
of  the  liquid  in  transmitted  light.  To  ascertain  if  this  is 
due  to  disperse  particles  and  not  to  fluorescence,  it  is  only 
necessary  to  view  the  cone  through  some  form  of  analyzer, 
e.g.,  a  Nicoll  prism  (See  Fig.  15.)  The  light  due  to 
fluorescence  is  not  polarized  (compare  p.  14.),  whereas 
dispersed  light  is  and  should  therefore  be  extinguished  in 
two  positions  of  the  Nicoll.  The  angle  of  the  plane  of 
polarization  with  the  axis  of  the  cone  depends  on  the 
natur  of  the  particles  :  if  they  are  electrical  non-con- 


v\ 
A\ 

\V 
V\ 


FIG.  15. 

ductors,  the  angle  is  a  right  one,  and  the  Nicoll  should  be 
placed  as  shown  in  full  lines.  With  metallic  particles  the 
angle  is  less  than  90  per  cent,  and  must  be  foun i  by  trial 
until  the  maximum  effect  is  obtained,  say  in  the  dotted 
position  of  the  analyzer. 

The  classical  and  conclusive  optical  method  of 
examination  is  of  course  that  by  the  ultra-microscope. 
The  slit  ultra-microscope  is  not  very  generally  available, 
nor  is  the  necessary  familiarity  with  high  power 


JKXTZSCH  S   CONDENSER.  JKo 

microscopy.  For  most  diagnostic  purposes,  however;  one 
of  the  dark  ground  condensers  already  referred  to  is 
likely  to  be  adequate,  if  a  sufficiently  powerful  source  of 
light  is  available.  These  involve  the  use  of  slides  and 
cover  glasses,  which  must  be  perfectly  clean  :  the  pro- 
cedure familiar  to  the  bacteriologist  (cleaning  with  hot 
bichromate-sulphuric  acid  mixture,  washing  with  distilled 
water  and  subsequently  with  alcohol,,  in  which  the  slides 
and  cover  glasses  are  kept  until  required,  when  the 
adherent  alcohol  is  burned  off)  is  suitable  for  the  purpose. 


FHJ.  16. 

A  more  recent  type,,  which  dispenses  with  the  use  of 
slides  and  gives  extremely  powerful  illiumination,  is  the 
double  reflection  condenser  designed  by  Jentzsch  (Fig. 
16).  The  liquid  is  placed  directly  in  the  spherical  hollow 
a,  which  is  closed  by  a  quartz  cover.  Exact  centering  is 
not  so  important  with  this  type  as  with  those  depending 
on  total  reflection,  which  is  a  great  convenience  in  use. 
The  depth  from  the  cover  to  the  focus  lying  within  the 
hollow  which  holds  the  liquid  makes  it  impossible  to  use  an 
objective  of  shorter  focus  than  4 ',  but  there  is  no  limit  to 
the  eye  piece  magnification  which  may  be  employed.  A 
good  i  objective  and  an  1 S  eyepiece  (Zeiss)  form  a 


96  DIALYSIS.        COLLODION   THIMBLES. 

combination  sufficient  for  all  purposes.  It  is  necessary  to 
use  an  arc  light  as  illuminant — a  hand  regulated  arc 
taking  about  5  amperes  being  suitable — and  a  water  cell 
should  be  interposed  between  it  and  the  mirror  of  the 
microscope.  It  is  hardly  necessary  to  add  that  this 
apparatus  cannot  be  used  for  the  measurement  of 
particles,  as  it  is  not  possible  to  observe  a  definite 
measured  volume.  It  is  on  the  other  hand  eminently 
convenient  for  making  coagulation  experiments. 

Dialysis,  as  already  explained,  is  the  fundamental 
method  of  colloidal  chemistry,  and  has  therefore  been 
treated  in  some  detail.  In  addition  to  parchment  paper, 
gold  beater's  skin  and  collodion  are  the  most  frequently 
sued  materials,  the  latter  generallj'  in  the  form  of 
thimbles.  The  collodion  used  is  the  sol  of  nitrocellulose 
(collodion  cotton)  in  ether  alcohol  and  is  prepared  by 
first  soaking  the  cotton  for  ten  to  fifteen  minutes  in 
ether  and  then  adding  alcohol  amounting  to  one  third  of 
the  volume  of  ether.  The  most  suitable  concentration  is 
about  3  per  cent.,  viz.,  three  grammes  of  cotton  to  75cc 
of  ether  and  25  cc  of  alcohol.  Various  directions  for 
making  the  thimbles  are  given  in  the  literature,  the  only 
difference  in  principle  being  that  they  can  be  formed 
either  on  the  outside  or  011  the  inside  of  the  test  or 
boiling  tubes  usually  employed  as  moulds.  The  author 
prefers  the  latter  method,  which  is  as  follows  :  a  perfectly 
clean  and  smooth  test  tube  is  partly  filled  with  collodion 
and  the  latter  poured  out  while  the  tube  is  being 
constantly  turned,  so  that  the  whole  internal  surface  is 
uniformly  coated.  When  the  collodion  has  dried  so  far  that 
it  no  longer  sticks  to  the  finger,  the  tube  is  placed  in  water 
at  about  37°,  when  the  film  gradually  shrinks  away  from 
the  glass  and  can  be  gently  withdrawn  after  a  few  minutes. 


ULTRA-FILTERS.  97 

Ultra-filtration  under  pressure  calls  for  special 
apparatus  and  even  apart  from  this  is  a  method  which  will 
be  called  for  only  in  very  special  cases.  The  form  of 
ultra-filter  used  by  the  author  is  shown  in  Fig.  17,  in 
sectional  elevation.  The  filtering  membrane  rests  on  a 
perforated  metal  disc  a,  which  is  clamped  between 


FIG.  17. 

the  body  b  of  the  filter  and  the  slightly  conical 
bottom  c  and  rests  on  six  radial  ribs  in  the  latter.  The 
branch  d,  closed  by  a  screw  cap,  serves  for  filling  the 
filter  and  the  necessary  pressure  is  generated  by  a 
bicycle  tyre  pump  connected  to  the  valve  e. 


98  ULTRA-FILTERS.    ELECTROLYTE    COAGULATION 

The  most  convenient  membranes  are  made  of 
hardened  filter  paper  impregnated  with  acetic  acid 
collodion.  Collodion  cotton  is  allowed  to  swell  in  a  smal] 
quantity  of  glacial  acetic  acid,  and  is  then  dissolved  by 
adding  an  amount  of  acid  sufficient  to  produce  a  1  to  5 
per  cent,  solution.  The  impregnation  of  the  circular  paper 
discs,  to  be  quite  perfect,  should  be  carried  out  in  vacuo  ; 
this  is  a  somewhat  complicated  matter,  and  satisfactory 
membranes  can  be  obtained  by  pouring  some  of  the 
collodion  into  a  shallow  dish  and  gradually  submerging  the 
paper,  held  at  a  small  angle  with  the  horizontal,  so  that 
no  air  bubles  are  trapped  under  it.  The  paper  should  be 
grasped  with  a  forceps  so  near  the  edge  that  the  point 
held,  which  is  likely  to  be  defective,  falls  within  the 
ioint  face  of  the  apparatus.  The  paper  is  then  withdrawn, 
and,  after  the  excess  of  collodion  has  been  allowed  to 
drain  off,  submerged  in  water.  Washing  either  witli 
repeated  changes  or  with  running  water  must  be  con- 
tinued until  all  acetic  acid  has  been  washed  out,  and  the 
prepared  discs  can  then  be  indefinitely  kept  uiidei  water. 
When  discs  are  made  with  different  concentrations  of 
collodion,  it  is  advisable  to  mark  the  percentage  strength 
of  the  latter  in  pencil  on  the  papers  before  impregnation. 
For  further  details  the  reader  must  be  referred  to  the 
original  papers  by  Bechhold  and  others. 

When  a  given  liquid  is  suspected  of  being  a  suspensoid 
it  will  be  natural  to  examine  its  behaviour  towards 
electrolytes.  What  has  been  said  in  Chapter  IV  about 
the  effect  of  hydrogen  ion,  the  influence  of  valency,  etc., 
should  be  borne  in  mind.  By  using  salts  of  trivalent 
cations,  e.g.,  aluminium  sulphate,  it  will  be  easy  to  find  in  a 
short  time  whether  any  marked  change  in  appearances 
takes  place  ;  even  if  this  is  the  case  sedimentation  may 


COAGULATION  AND  CATAPHORESIS  TESTS.  99 

yet  be  slow,  as  the  rate  depends  on  the  difference  in  the 
specific  gravities  of  the  phases.  If  it  is  desired  to 
ascertain  approximately  the  limit  concentration  at  which 
coagulation  takes  place,  it  is  advisable  to  refer  to  the 
values  given  in  Chapter  IV,  so  as  to  have  some  idea  of  the 
order  of  concentration.  A  number  of  samples  of  say  1 0  cc 
are  then  taken,  and  say  1  cc  of  a  suitable  electrolyte 
solution  added  to  the  first,  2  cc  to  the  second  sample, 
etc.  The  concentration  of  the  solution  must  be  so  chosen 
that  the  electrolyte  concentration  in  the  first  sample  is 
well  below,  and  that  in  the  last  sample  well  above,  the 
probable  value.  The  samples  are  then  allowed  to  stand 
for  some  time.  When  no  further  alteration  occurs,  the 
concentration  in  the  limit  value  obviously  lies  somewhere 
between  the  concentration  in  the  first  sample  showing 
any  alteration  (change  of  colour,  turbidity)  and  that  in 
the  last  sample  which  fails  to  do  so. 

Cataphoresis  tests  may  also  be  tried,  and,  where  the 
electric  charge  on  the  disperse  phase  is  of  doubtful  sign, 
may  precede  coagulation  trials.  The  apparatus  used  for 
this  purpose  has  already  been  illustrated  (Fig.  6,  p.  29.) 
It  is  charged  in  the  following  manner :  a  rubber  tube 
15 — 18  long  is  attached  to  the  central  branch  of  the 
U-tube,  a  small  funnel  being  inserted  into  the  other  end 
of  the  rubber  tube.  The  cock  is  now  opened,  the  funnel 
held  level  with  it  and  the  sol  poured  into  the  latter  until 
it  appears  at  the  cock.  The  funnel  is  then  raised  and 
lowered  a  few  times  to  expel  any  air  which  may  have 
become  trapped  and  is  finally  raised  slowly  until  the 
liquid  reaches  the  bore  of  the  cock,  which  is  then  closed. 
The  U-tube  is  now  half  filled  with  distilled  water,  the 
funnel  raised  till  the  liquid  in  it  is  exactly  level  with  the 
water  in  the  U-tube  and  the  cock  opened.  The  funnel 


100  PREPARATION    OF    GELATINE     AND     AGAK     SOLS. 

is  then  raised  slowly  and  uniformly  until  the  sol  has 
risen  into  both  limbs  and  the  water  above  it  reaches  the 
electrodes.  By  careful  procedure  it  is  easy  to  obtain  a 
sharp  boundary ;  should  any  difficulty  be  experienced, 
the  sol  can  be  loaded  with  a  non-electrolyte,  e.g.,  sugar. 
The  apparatus  may  be  connected  to  the  electric  light 
supply,  if  the  distilled  wrater  is  reasonably  pure,  and  it 
will  generally  be  found  easier  to  determine,  after  a  short 
time,  which  boundaiy  recedes  from  the  electrode  rather 
than  the  opposite.  If  the  result  appears  doubtful,  or  if 
the  boundary  has  become  blurred,  it  is  advisable  to 
reverse  the  polarity  of  the  electrodes  a  few  times,  until 
the  result  is  unambiguous. 

As  regards  methods  of  preparation,  various  examples 
of  inorganic  suspensoid  sols  have  already  been  given, 
extensions  of  which  to  other  substances  will  suggest 
themselves.  For  a  full  account  of  the  subject  the  reader 
is  referred  to  Svedberg's  work  on  the  preparation  of 
inorganic  sols. 

The  preparation  of  the  most  typical  organic 
ernulsoid  sols,  gelatin  and  agar.,  presents  no  difficulties 
beyond  the  inherent  one  of  getting  exact  duplicate 
results.  To  ensure  these  as  far  as  possible,  it  is 
necessary  to  start  which  a  batcli  of  raw  material 
sufficient  for  any  given  investigation,  and  to  adhere 
rigidly  to  the  procedure  once  chosen  as  regards  times  and 
temperatures. 

Both  substances  should  be  allowed  to  swell  fully  in 
cold  water  before  they  are  dispersed  :  gelatin  for  two  or 
three,  and  agar  for  about  24  hours.  In  the  latter  case  it  is 
usual  to  add  to  the  water  a  trace — say  one  part  in  500 — 
of  acetic  acid,  which  promotes  imbibition.  Gelatin 
should  then  be  warmed  to  not  more  than  50  ,  while  agar 


PREPARATION  OF  SILICIC  ACID  SOLS.  101 

requires  boiling.  If  clear  sols  or  gels  are  wanted,  both 
will  require  filtering  on  hot  water  funnels ;  in 
bacteriological  practice  Chardin  paper,  which  can  be 
obtained  in  sheets  or  in  the  shape  of  large  folded  filters, 
is  usually  employed  for  the  purpose.  Where  reproducible 
results  only  are  required,  as  for  instance  in  the  study  of 
Liesegang's  or  similar  phenomena,  it  will  be  found  much 
easier  to  make  sols  which  contain  a  definite  percentage  of 
solid  to  a  given  volume  of  water  rather  than  in  a  given 
volume  of  sol,  as  the  preparation  of  the  latter  is 
complicated  by  the  previous  swelling. 

The  only  inorganic  emulsoid  sol  which  may  require 
preparation  in  quantities  is  silicic  acid.  A  stock  solution 
of  sodium  silicate  should  be  prepared  having  a  specific 
gravity  of  1*16 — 1*18,  and  kept  in  bottle  fitted  with  a 
rubber  stopper,  as  glass  stoppers  are  attacked  and  stick. 
One  kilogramme  of  the  commercial  water  glass  syrup  to 
about  5  litres  of  water  will  give  approximately  the 
strength  specified  ;  the  water  should  be  free  from  carbon 
dioxide.  To  prepare  a  sol,  75  cc  of  this  silicate 
solution  is  poured  into  a  mixture  of  25  cc  of  concentrated 
hydrochloric  acid  and  75  cc  of  water  and  the  whole 
dialysed  against  running  or  frequently  changed  water. 
Experience  will  show  how  far  dialysis  may  be  carried 
safely  without  getting  gel  formation  in  the  dialyser.  If  a 
gel  is  required,  a  small  quantity  of  very  dilute  ammonia 
should  be  added  to  the  the  sol  a  few  hours  before  the 
gel  is  wanted. 

Experimental  work  on  adsorption  forms  the  most 
difficult  branch  of  the  subject  and  it  is  hardly  possible  to 
give  general  rules,  since  the  principal  problem  is  to  find 
in  any  given  case  a  method  of  analysis  which  permits  the 
determination  of  small  differences  between  concentrations 


102 


ADSORPTION    EXPERIMENTS. 


which  are  already  of  a  very  small  order.  Valuable  hints 
on  procedure  will  be  found  in  the  various  papers  published 
by  Freundlich  and  his  pupils.  As  a  simple  example,  or  as 
a  lecture  experiment.,  the  adsorption  of  oxalic  acid  by 
charcoal' may  be  determined,  and  gives  fairly  concordant 
and  typical  results.  A  solution  of  20  grammes  in  500  cc 
is  prepared  and  100  cc  set  aside.  From  the  rest  five  lots 
of  100  cc  each  are  prepared  in  which  the  concentrations, 
taking  the  original  sol  as  6,  are  5,  4,  3,  2  and  1  respectively. 
One  gramme  of  powdered  charcoal  is  placed  in  each 
sample,  shaken  up  several  times,  and  finally  allowed  to 
settle  for  24  hours  to  avoid  filtration  and  the  consequent 
errors  due  to  adsorption  by  filter  paper.  10  cc  of  each 
sample  is  then  titrated  with  permanganate  and  sulphuric- 
acid  in  the  usual  way.  As  regards  the  strength  of 
permanganate  to  be  used  the  following  consideration 
must  be  borne  in  mind  :  in  the  last  sample  with  the 
concentration  one — i.e.,  0.666  gr.  per  100  cc — practically 
the  whole  of  the  acid  will  be  adsorbed.  The  remaining 
acid  should,  however,  still  be  determined  with  reasonable 
exactness,  so  that  a  very  dilute  permanganate  solution 
must  be  used,  say  one  of  which  15  to  20  cc  will  be 
required  for  the  sample  1.  before  adsorption. 

As  the  scale  on  which  the  results  are  plotted  is 
arbitrary,  the  number  of  cc.s  of  permanganate  may  be 
used.  The  number  of  cc.s  used  for  each  sample  after  ad- 
sorption represents  the  end  or  equilibrium  concentration 
which  is  plotted  as  abscissa,  while  the  difference  between 
this  number  and  the  original  concentration  of  the  sample, 
expresssed  in  cc  of  permanganate,  is  the  amount  adsorbed 
and  is  therefore  plotted  as  ordinate.  If  the  work  is 
carefully  done  and  if  the  charcoal  used  is  fairly  uniform,  a 
very  satisfactory  and  typical  parabola  is  obtained. 


SUBJECT-MATTER     INDEX. 

A. 

PAGE 

Adsorption     ......  6,  68 

and  chemical  changes        .           .           .           .  .82 

compounds    .                      .  .57 
of  dissociated   substances           .           .           .75,  86 

experiments  .                                             .  101 
electrical        .           .                      ....    85 

equilibrium    .                                  .           .  .76 

isotherm         .  .....    77 

negative                     .           .           .           .           .  .73 

selective          .           .           .           .           .           .  .6 

Agdr  sols        .                                  .  44,  100 

gels 44,  54 

Albumin  sols            .           .           .           .           .           .           .  ,     .    47 

electrolyte  coagulation  of  .47 

heat  coagulation  ol"         .           .                      .  .    47 

and  Hofmeister  series    .  .48 

highly  purified     .           .           .           .           .  .51 

Aluminium  hydroxide  (for  adsorption)           .           .           .  .75 

sol           .                      ...  2,  3 

Amicrons        .....                      .  .18 

Antimony   trisulphide    sol           ....  .9 

Arsanic  trisulphide  sol       .           .           .           .           .           .  .9 

B. 

Brownian  movement        .  25 — 27 


Cadmium  sulphide  sol        .....  .33 

Chromium  hydroxide  sol                        .           .  .2 

Congo  red,  adsorption  of                        .  .82 

size  of  molecule       .           .           .           .  .           .11 

Continuous  phase    ......  .19 

Crystalloids    ...                      .  .1 

D. 

Dialysis          .                     ,                                           .  1,  11 

apparatus  for       .                      .                      .  .6  —  8.96 

Diffusion  in  gels      .  .  .  ...    63 

Disperse  phase        .           .          *..  -.           .19 


104 

E. 

PACK 

Electric  charge  on  disperse  phase        .           .           .           .  .27 

influence  of  reaction  of  medium      .           .  .28 

methods  of  demonstration     .           .           .  29,  99 

Electrolytes,  action  on  suspensoids       ....  2,  33 

effect  of  valency  of  cation         .           .           .  .34 

Freundlich's  theory  of  action    .           .           .  .80 

Hofmeister's  series  of                .           .           .  .    48 

quantities  taken  down  by  gels           .           .  .35 

Emulsifying  agents            .           .           .           .           .           .  .41 

effect  on  interfacial  tension       .           .  .41 
Emulsions     .........    38 

pure  oil-water            .....  38,  39 

rich  in  disperse  phase         .  .  .  .22,  39 

viscosity  of                 .           .           .           .           .  22.  42 

Emulsoids      .           .           .           .                      .           .           .  44,  51 

electrical  properties  of                   .           .           .  .51 

viscosity  of    .           .             .           .           .  22.  45 

ultra-microscopic  image  of            .           .           .  .51 

Exponents  of  adsorption  isotherm 

Extraction  curves    ......  . 

relation  with  adsorption  isotherm         .  .    84 

F. 

Ferric  hydroxide  sol          .           .           .           .                     .  2,  3,  9 

G. 

d-Galactan     .          .           .          .          .          .          .          .  .45 

Gels      .          .       < .  5,54—67 

elastic      .           .           .                                .           .           .  .54 

diffusion  in                  .           .           .           .           .          '.  .63 

reactions  in                 .           .           .           .           .           .  .    (-54 

rigid        .          .          ;          .          .          .          .          .  .54 

structure  of                 .           .           ;,  .       .           .           .  61,  68 

Gelatine  sols            .          .          .          .          .          .  45,  100 

Gold  figures  .  ......    37 

sols     .  .                                .  3,  8 

H. 

Hysteresis  (of  setting  temperature)       .           .           .           .  .45 

I. 

India  rubber,  latex  ........    38 

thermal  expansion  of                  .           .           .  .61 

L. 

Liesegang's  rings    ........    64 

Lyophilic 

Lyophobic      .........    20 


M. 

Mercury  sols  ........      3 

Methods  of  preparing  suspensoid  sols  .                      .      8,  9,  31 

Microns          .....  ...     18 

O. 

Oscillating  discharge                    ,  .32 

Osmotic  pressure  of  sols  .  ....      5 

P, 

Parchment  thimbles           .  .8 

tubes    ...  7 

Peptisation     ....  .33 

Platinum  sols           ...  ....    32 

Protective  colloids    ....  .           .    36 

Silicic  acid  gel          .           .  2,  54.  101 

sol          ...  2,  43.    101 

Silver  sol       ....  ....       9 

Size  of  particles       ...  14,  1 

Sols ....      2 

Surface  energy        ...  ,                                     69 

tension       ....  .         40,  69 

Suspended  particle,  motion  of    .           .  .    24 
Suspensions              ........    22 

Suspensoids  ........    23 

Swelling  of  elastic  gels     .  .           .           .    5H 

T. 

Tyndall  illumination  10,  93 


NAME     INDEX. 


A. 


Ambronn,  15 


Bachmann,  62 

Bayliss,  82 
Bechhold  12,  36,  63 
Bemmelen,  van,  56,  75 
Benson,  72 
Bredig,  32 
Burton,  28,  33 
Biitschli,  62 


c, 


Cotton.  28 
Cram,  3 


Davis,  85 
Dornan,  41 
Dreaper,  85 


Einstein,  22,  27 


F. 


Faraday.  3,  35 
Fi/eau,  15 

Freundlich,  20.  34,  59v  76,  78,  80, 
101 

G. 

Garrett.  45 
Gibbs,  72 
Goppelsroeder,  81 
Graham,  1,  5,  33,  44,  63 


H. 

Hardy,  28.  33 
Hatschek,  22,  39,  65 
Hofmeister.  48 


Kuhn,  3 
Kuzel,  32 


Liesegang,  64 
Linder,  12,  33 


K 


L. 


M. 


Mengarini-Taube,  33 
Mouton,  28 


O. 

Ostvvald,  Wilhelm,  1,  76 
Ostvvald,  Wolfgang,  20,  23, 
53,  86 

P. 

Paal,  3,  36 
Pauli,  47,  48,  51 
Pean  de  St.  Gilles,  3 
Perrin.  20,  27 
Pickering,  41,  43 
Picton,  12,  33 


0 


Quincke,  21 


R. 


Kayleigli,  14 
Keinke,  59 


Scala,  33 
Sc breeder,  58 
Siedentopf,  3,  26 
Smoluehowski,  27 

Soddy,  27 
Spence,  83 
Stokes,  23 
Svedheru,  e,  32 


107 


T. 

Tyndall,  10,  61,  93 
\V. 


Walden,  3 
Wislicenus,  75 


Ziegler,  63 
Zsigmondy.  3,  26.  36,  62 


THIS  BOOK  IS  DUE  ON  THE 


UNTVE 


LAST 


oAERDAuNED  T°  ~*T°°°°°»  ^s^"^? 


SEP  2*  1333 

I 

FEB  18  1937 


MAR    1  1935 
SEP  13 1935 


MAR   15   1938 
JAN    161945 


JUN  2  2  1951 


LD  2l-50m-l,'3J 


YB   i669 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


